diff --git a/gdd.md b/gdd.md
index fa2b769..b780dcd 100644
--- a/gdd.md
+++ b/gdd.md
@@ -11,15 +11,14 @@
 ## Clock
 
 Ticks can be applied manually or they can be applied over time.
-Add a tick counter as a UI element.
+There is a tick counter UI debug element.
 
-## Grid
+## Diffusion
+
+### Grid
 
 Add a 10x10 grid.
 Tiles are highlighted a different colour when the player hovers over them with the cursor.
-
-### Diffusion
-
 Tiles in the grid contain floating point values.
 These values propogate to neighbouring cells a tick happens.
 
diff --git a/half-earth/resources/overlays/no_overlay.tres b/half-earth/resources/overlays/no_overlay.tres
index d4a783f..5f79807 100644
--- a/half-earth/resources/overlays/no_overlay.tres
+++ b/half-earth/resources/overlays/no_overlay.tres
@@ -4,3 +4,5 @@
 
 [resource]
 script = ExtResource( 1 )
+_healthy = Color( 0.0431373, 0.541176, 0.0235294, 1 )
+_depleted = Color( 0.670588, 0.572549, 0.27451, 1 )
diff --git a/half-earth/resources/tiles/tile_types/developed.tres b/half-earth/resources/tiles/tile_types/developed.tres
index 8f5f1d1..d9ffdac 100644
--- a/half-earth/resources/tiles/tile_types/developed.tres
+++ b/half-earth/resources/tiles/tile_types/developed.tres
@@ -7,4 +7,4 @@ script = ExtResource( 1 )
 Name = "Developed"
 Key = 68
 Color = Color( 0.372549, 0.372549, 0.372549, 1 )
-HeatGeneration = 0.4
+HeatGeneration = 0.15
diff --git a/half-earth/resources/tiles/tile_types/tile_type_collections/buildable_tiles.tres b/half-earth/resources/tiles/tile_types/tile_type_collections/buildable_tiles.tres
index af140a6..1656e8c 100644
--- a/half-earth/resources/tiles/tile_types/tile_type_collections/buildable_tiles.tres
+++ b/half-earth/resources/tiles/tile_types/tile_type_collections/buildable_tiles.tres
@@ -1,8 +1,9 @@
-[gd_resource type="Resource" load_steps=3 format=2]
+[gd_resource type="Resource" load_steps=4 format=2]
 
 [ext_resource path="res://scripts/tile/TileTypeCollection.cs" type="Script" id=1]
 [ext_resource path="res://resources/tiles/tile_types/developed.tres" type="Resource" id=2]
+[ext_resource path="res://resources/tiles/tile_types/wild.tres" type="Resource" id=3]
 
 [resource]
 script = ExtResource( 1 )
-_resources = [ ExtResource( 2 ) ]
+_resources = [ ExtResource( 2 ), ExtResource( 3 ) ]
diff --git a/half-earth/resources/tiles/tile_types/wild.tres b/half-earth/resources/tiles/tile_types/wild.tres
index 370bb83..ed76163 100644
--- a/half-earth/resources/tiles/tile_types/wild.tres
+++ b/half-earth/resources/tiles/tile_types/wild.tres
@@ -1,10 +1,12 @@
 [gd_resource type="Resource" load_steps=2 format=2]
 
-[ext_resource path="res://scripts/tile/TileType.cs" type="Script" id=1]
+[ext_resource path="res://scripts/tile/Wild.cs" type="Script" id=1]
 
 [resource]
 script = ExtResource( 1 )
 Name = "Wild"
 Key = 87
-Color = Color( 0, 0.545098, 0.0196078, 1 )
-HeatGeneration = -0.2
+Color = Color( 0.14902, 1, 0, 0 )
+HeatGeneration = -0.02
+HealRate = 0.01
+Threshold = 0.6
diff --git a/half-earth/scripts/EnvironmentSystem.cs b/half-earth/scripts/EnvironmentSystem.cs
new file mode 100644
index 0000000..5fabf8b
--- /dev/null
+++ b/half-earth/scripts/EnvironmentSystem.cs
@@ -0,0 +1,117 @@
+using Godot;
+
+public class EnvironmentSystem
+{
+    private readonly TileGrid _tiles;
+    private readonly float _diffusionCoefficient;
+
+    private float[] _nextTemperatures = null;
+
+    public EnvironmentSystem(TileGrid tiles, float diffusionCoefficient)
+    {
+        _tiles = tiles;
+        _diffusionCoefficient = diffusionCoefficient;
+        _nextTemperatures = new float[tiles.Count];
+    }
+
+    public void Process()
+    {
+        GenerateHeat();
+        Diffuse();
+    }
+
+    private float TransferHeat(float t0, float alpha, float nx, float ny, float px, float py)
+    {
+        float d2tdx2 = nx - 2 * t0 + px;
+        float d2tdy2 = ny - 2 * t0 + py;
+        return t0 + alpha * (d2tdx2 + d2tdy2);
+    }
+
+    private void GetNeighbourTemperatures(int x, int y, out float nx, out float ny, out float px, out float py)
+    {
+        // default value
+        var t = _tiles[x, y].temperature;
+
+        nx = x > 0 ? _tiles[x - 1, y].temperature : t;
+        px = x < _tiles.Size - 1 ? _tiles[x + 1, y].temperature : t;
+
+        ny = y > 0 ? _tiles[x, y - 1].temperature : t;
+        py = y < _tiles.Size - 1 ? _tiles[x, y + 1].temperature : t;
+    }
+    private void GenerateHeat()
+    {
+        for (int i = 0; i < _tiles.Count; i++)
+        {
+            var tile = _tiles[i];
+            var type = tile.type;
+            tile.temperature += type.HeatGeneration;
+            _tiles[i] = tile;
+        }
+
+        for (int i = 0; i < _tiles.Count; i++)
+        {
+            ApplyHeatDamage(i);
+        }
+    }
+
+    private void ApplyHeatDamage(int i)
+    {
+        _tiles.MapIndex(i, out var x, out var y);
+        var tile = _tiles[x, y];
+        if (!(tile.type is IDamageable damageable))
+            return;
+
+        var delta = tile.temperature - damageable.Threshold;
+        if (delta < 0)
+        {
+            // we want to heal based on how many wild tiles surround us
+            float wild = _tiles.GetNeighbourProportion<Wild>(x, y);
+            float heal = damageable.HealRate * wild;
+
+            tile.currentHealth += heal;
+        }
+        else
+        {
+            // TODO: parameterised balancing
+
+            // take damage
+            tile.currentHealth -= delta;
+        }
+
+        tile.currentHealth = Mathf.Clamp(tile.currentHealth, 0, 1);
+
+        _tiles[x, y] = tile;
+    }
+
+    private void Diffuse()
+    {
+        var D = _diffusionCoefficient;
+
+        for (int i = 0; i < _tiles.Count; i++)
+        {
+            float t = _tiles[i].temperature;
+
+            _tiles.MapIndex(i, out var x, out var y);
+
+            GetNeighbourTemperatures(
+                x, y,
+                out var nx,
+                out var ny,
+                out var px,
+                out var py);
+
+            var temperature = TransferHeat(t, D, nx, ny, px, py);
+
+            // TODO: what if it's really really cold out?
+            temperature = Mathf.Max(0, temperature);
+            _nextTemperatures[i] = temperature;
+        }
+
+        for (int i = 0; i < _tiles.Count; i++)
+        {
+            var tile = _tiles[i];
+            tile.temperature = _nextTemperatures[i];
+            _tiles[i] = tile;
+        }
+    }
+}
diff --git a/half-earth/scripts/WorldGrid.cs b/half-earth/scripts/WorldGrid.cs
index 6a7b01a..10a7250 100644
--- a/half-earth/scripts/WorldGrid.cs
+++ b/half-earth/scripts/WorldGrid.cs
@@ -16,13 +16,12 @@ public class WorldGrid : Node2D
     [Export]
     public float DiffusionCoefficient { get; set; } = 0.01f;
 
-    private float[] _nextValues = null;
-
     public struct TileView
     {
         public ShaderMaterial material;
     }
     private TileView[] _tileViews;
+    private EnvironmentSystem _environment;
 
     // Called when the node enters the scene tree for the first time.
     public override void _Ready()
@@ -30,6 +29,8 @@ public class WorldGrid : Node2D
         _tileGrid = (TileGrid)_tileGridResource;
 
         GenerateCanvasItems(_tileGrid);
+
+        _environment = new EnvironmentSystem(_tileGrid, DiffusionCoefficient);
     }
 
     #region Positioning
@@ -52,9 +53,8 @@ public class WorldGrid : Node2D
 
         var tile = _tileGrid[idx];
         tile.type = tileType;
+        tile = tileType.ApplySpawneffect(tile);
         _tileGrid[x, y] = tile;
-
-        _tileViews[idx].material.SetShaderParam("lowColor", tileType.Color);
     }
     #endregion
 
@@ -76,7 +76,6 @@ public class WorldGrid : Node2D
         this.Clear();
 
         _tileViews = new TileView[grid.Count];
-        _nextValues = new float[grid.Count];
 
         for (int i = 0; i < grid.Count; i++)
         {
@@ -96,80 +95,11 @@ public class WorldGrid : Node2D
 
             TileView view = default;
             view.material = (ShaderMaterial)canvasItem.Material;
-            view.material.SetShaderParam("lowColor", _tileGrid.StartTileType.Color);
             _tileViews[i] = view;
         }
     }
 
     public ShaderMaterial GetTileMaterial(int idx) => _tileViews[idx].material;
 
-    #region Simulation
-    public void _on_Clock_OnTick(int ticks)
-    {
-        GenerateHeat();
-        Diffuse();
-    }
-
-    private float TransferHeat(float t0, float alpha, float nx, float ny, float px, float py)
-    {
-        float d2tdx2 = nx - 2 * t0 + px;
-        float d2tdy2 = ny - 2 * t0 + py;
-        return t0 + alpha * (d2tdx2 + d2tdy2);
-    }
-
-    private void GetNeighbourTemperatures(int x, int y, out float nx, out float ny, out float px, out float py)
-    {
-        // default value
-        var t = _tileGrid[x, y].temperature;
-
-        nx = x > 0 ? _tileGrid[x - 1, y].temperature : t;
-        px = x < _tileGrid.Size - 1 ? _tileGrid[x + 1, y].temperature : t;
-
-        ny = y > 0 ? _tileGrid[x, y - 1].temperature : t;
-        py = y < _tileGrid.Size - 1 ? _tileGrid[x, y + 1].temperature : t;
-    }
-
-    private void GenerateHeat()
-    {
-        for (int i = 0; i < _tileGrid.Count; i++)
-        {
-            var tile = _tileGrid[i];
-            var type = tile.type;
-            tile.temperature += type.HeatGeneration;
-            _tileGrid[i] = tile;
-        }
-    }
-
-    private void Diffuse()
-    {
-        var D = DiffusionCoefficient;
-
-        for (int i = 0; i < _tileGrid.Count; i++)
-        {
-            float t = _tileGrid[i].temperature;
-
-            _tileGrid.MapIndex(i, out var x, out var y);
-
-            GetNeighbourTemperatures(
-                x, y,
-                out var nx,
-                out var ny,
-                out var px,
-                out var py);
-
-            var temperature = TransferHeat(t, D, nx, ny, px, py);
-
-            // TODO: what if it's really really cold out?
-            temperature = Mathf.Max(0, temperature);
-            _nextValues[i] = temperature;
-        }
-
-        for (int i = 0; i < _tileGrid.Count; i++)
-        {
-            var tile = _tileGrid[i];
-            tile.temperature = _nextValues[i];
-            _tileGrid[i] = tile;
-        }
-    }
-    #endregion
+    public void _on_Clock_OnTick(int ticks) => _environment.Process();
 }
diff --git a/half-earth/scripts/overlays/NoOverlay.cs b/half-earth/scripts/overlays/NoOverlay.cs
index 49f8b1b..1222cec 100644
--- a/half-earth/scripts/overlays/NoOverlay.cs
+++ b/half-earth/scripts/overlays/NoOverlay.cs
@@ -2,10 +2,25 @@ using Godot;
 
 public class NoOverlay : Overlay
 {
+    [Export]
+    private Color _healthy;
+
+    [Export]
+    private Color _depleted;
+
     // in this view we want to draw tiles with their normal colour
     public override void Apply(Tile tile, ShaderMaterial material)
     {
         var type = tile.type;
+
+        if (type is Wild wild)
+        {
+            material.SetShaderParam("lowColor", _depleted);
+            material.SetShaderParam("highColor", _healthy);
+            material.SetShaderParam("t", tile.currentHealth);
+            return;
+        }
+
         material.SetShaderParam("lowColor", type.Color);
         material.SetShaderParam("t", 0);
     }
diff --git a/half-earth/scripts/tile/IDamageable.cs b/half-earth/scripts/tile/IDamageable.cs
new file mode 100644
index 0000000..ba606f2
--- /dev/null
+++ b/half-earth/scripts/tile/IDamageable.cs
@@ -0,0 +1,5 @@
+public interface IDamageable
+{
+    float HealRate { get; }
+    float Threshold { get; }
+}
\ No newline at end of file
diff --git a/half-earth/scripts/tile/Tile.cs b/half-earth/scripts/tile/Tile.cs
index 662c72c..3a51b55 100644
--- a/half-earth/scripts/tile/Tile.cs
+++ b/half-earth/scripts/tile/Tile.cs
@@ -5,4 +5,5 @@ public struct Tile
     public bool isHighlighted;
     public float temperature;
     public TileType type;
+    public float currentHealth;
 }
\ No newline at end of file
diff --git a/half-earth/scripts/tile/TileGrid.cs b/half-earth/scripts/tile/TileGrid.cs
index 7a2eb08..7ea685e 100644
--- a/half-earth/scripts/tile/TileGrid.cs
+++ b/half-earth/scripts/tile/TileGrid.cs
@@ -10,7 +10,7 @@ public class TileGrid : Resource, IReadOnlyList<Tile>
 
     [Export]
     private Resource _startTileTypeResource;
-    public TileType StartTileType => (TileType)_startTileTypeResource;
+    private TileType StartTileType => (TileType)_startTileTypeResource;
 
     public int Count => Size * Size;
 
@@ -58,6 +58,7 @@ public class TileGrid : Resource, IReadOnlyList<Tile>
             tile.isHighlighted = false;
             tile.temperature = 0.0f;
             tile.type = StartTileType;
+            tile.currentHealth = 1.0f;
 
             tiles.Add(tile);
         }
@@ -88,5 +89,64 @@ public class TileGrid : Resource, IReadOnlyList<Tile>
     {
         return x >= 0 && x < Size && y >= 0 && y < Size;
     }
+
+    private readonly int[] _neighbours = new int[]
+    {
+        -1, 0,
+        0, 1,
+        1, 0,
+        0, -1
+    };
+
+    private void GetCoordinate(int dir, out int x, out int y)
+    {
+        dir %= 4;
+        int idx = dir * 2;
+        x = _neighbours[idx];
+        y = _neighbours[idx + 1];
+    }
+
+    public float GetNeighbourProportion<T>(int x, int y)
+    {
+        int typed = GetNeighbourCount<T>(x, y);
+        int all = GetNeighbourCount(x, y);
+        return (float)typed / (float)all;
+    }
+
+    public int GetNeighbourCount<T>(int x, int y)
+    {
+        int count = 0;
+        for (int i = 0; i < 4; i++)
+        {
+            GetCoordinate(i, out var nx, out var ny);
+
+            if (!IsInBounds(x + nx, y + ny))
+                continue;
+
+            if (!(this[x + nx, y + ny].type is T))
+                continue;
+
+            count++;
+        }
+
+        return count;
+    }
+
+    public int GetNeighbourCount(int x, int y)
+    {
+        int count = 0;
+
+        for (int i = 0; i < 4; i++)
+        {
+            GetCoordinate(i, out var nx, out var ny);
+
+            if (!IsInBounds(x + nx, y + ny))
+                continue;
+
+            count++;
+        }
+
+        return count;
+    }
 }
 
diff --git a/half-earth/scripts/tile/TileType.cs b/half-earth/scripts/tile/TileType.cs
index c2824a1..f682ed1 100644
--- a/half-earth/scripts/tile/TileType.cs
+++ b/half-earth/scripts/tile/TileType.cs
@@ -17,4 +17,6 @@ public class TileType : Resource
     public float HeatGeneration { get; private set; }
 
     public override string ToString() => Name;
+
+    public virtual Tile ApplySpawneffect(Tile tile) => tile;
 }
\ No newline at end of file
diff --git a/half-earth/scripts/tile/Wild.cs b/half-earth/scripts/tile/Wild.cs
new file mode 100644
index 0000000..b058b44
--- /dev/null
+++ b/half-earth/scripts/tile/Wild.cs
@@ -0,0 +1,16 @@
+using Godot;
+
+public class Wild : TileType, IDamageable
+{
+    [Export]
+    public float HealRate { get; private set; } = 0.01f;
+
+    [Export]
+    public float Threshold { get; private set; }
+
+    public override Tile ApplySpawneffect(Tile tile)
+    {
+        tile.currentHealth = 0;
+        return tile;
+    }
+}
\ No newline at end of file
diff --git a/half-earth/scripts/ui/BuildModeUI.cs b/half-earth/scripts/ui/BuildModeUI.cs
index 92ddfa0..16461c1 100644
--- a/half-earth/scripts/ui/BuildModeUI.cs
+++ b/half-earth/scripts/ui/BuildModeUI.cs
@@ -20,10 +20,14 @@ public class BuildModeUI : Control
         _buildMode = GetNode<BuildMode>(_buildModePath);
 
         var buttonHeight = 40;
-        var tileType = _buildMode.BuildableTiles[0];
-        int x = 50;
 
-        SpawnButton(tileType, buttonHeight, ref x);
+        int x = 50;
+        int margin = 5;
+        foreach (var tt in _buildMode.BuildableTiles)
+        {
+            SpawnButton(tt, buttonHeight, ref x);
+            x += margin;
+        }
     }
 
     private void UpdateButtonToggleState(TileType tileType)