diff --git a/OpenRA.Game/DomainIndex.cs b/OpenRA.Game/DomainIndex.cs
new file mode 100644
index 0000000000..0207556e69
--- /dev/null
+++ b/OpenRA.Game/DomainIndex.cs
@@ -0,0 +1,106 @@
+using System;
+using System.Collections.Generic;
+using System.Drawing;
+using System.Linq;
+
+using OpenRA.Effects;
+using OpenRA.FileFormats;
+using OpenRA.Graphics;
+using OpenRA.Network;
+using OpenRA.Orders;
+using OpenRA.Support;
+using OpenRA.Traits;
+using XRandom = OpenRA.Thirdparty.Random;
+
+namespace OpenRA
+{
+	public class DomainIndex
+	{
+		Rectangle bounds;
+		int[,] domains;
+
+		public DomainIndex(World world)
+		{
+			bounds = world.Map.Bounds;
+			domains = new int[(bounds.Width + bounds.X), (bounds.Height + bounds.Y)];
+
+			BuildDomains(world);
+		}
+
+		public int GetDomainOf(CPos p)
+		{
+			return domains[p.X, p.Y];
+		}
+
+		public bool IsCrossDomain(CPos p1, CPos p2)
+		{
+			return GetDomainOf(p1) != GetDomainOf(p2);
+		}
+
+		public void SetDomain(CPos p, int domain)
+		{
+			domains[p.X, p.Y] = domain;
+		}
+
+		void BuildDomains(World world)
+		{
+			Map map = world.Map;
+
+			int i = 1;
+			HashSet<CPos> unassigned = new HashSet<CPos>();
+
+			// Fill up our set of yet-unassigned map cells
+			for (int x = map.Bounds.Left; x < bounds.Right; x += 1)
+			{
+				for (int y = bounds.Top; y < bounds.Bottom; y += 1)
+				{
+					unassigned.Add(new CPos(x, y));
+				}
+			}
+
+			while (unassigned.Count != 0)
+			{
+				var start = unassigned.First();
+				unassigned.Remove(start);
+
+				// Wander around looking for water transitions
+				bool inWater = WorldUtils.GetTerrainInfo(world, start).IsWater;
+				Queue<CPos> toProcess = new Queue<CPos>();
+				HashSet<CPos> seen = new HashSet<CPos>();
+				toProcess.Enqueue(start);
+
+				do
+				{
+					CPos p = toProcess.Dequeue();
+					if (seen.Contains(p)) continue;
+					seen.Add(p);
+
+					TerrainTypeInfo cellInfo = WorldUtils.GetTerrainInfo(world, p);
+					bool isWater = cellInfo.IsWater;
+
+					// Check if we're still in one contiguous domain
+					if (inWater == isWater)
+					{
+						SetDomain(p, i);
+						unassigned.Remove(p);
+
+						// Visit our neighbors, if we haven't already
+						foreach (var d in CVec.directions)
+						{
+							CPos nextPos = p + d;
+							if (nextPos.X >= map.Bounds.Left && nextPos.Y >= map.Bounds.Top &&
+								nextPos.X < map.Bounds.Right && nextPos.Y < map.Bounds.Bottom)
+							{
+								if (!seen.Contains(nextPos)) toProcess.Enqueue(nextPos);
+							}
+						}
+					}
+				} while (toProcess.Count != 0);
+
+				i += 1;
+			}
+
+			Log.Write("debug", "{0}: Found {1} domains", map.Title, i-1);
+		}
+	}
+}
diff --git a/OpenRA.Game/OpenRA.Game.csproj b/OpenRA.Game/OpenRA.Game.csproj
index 0aca544852..2c5c9cebbf 100644
--- a/OpenRA.Game/OpenRA.Game.csproj
+++ b/OpenRA.Game/OpenRA.Game.csproj
@@ -85,6 +85,7 @@
     <Compile Include="ActorInitializer.cs" />
     <Compile Include="ActorMap.cs" />
     <Compile Include="ActorReference.cs" />
+    <Compile Include="DomainIndex.cs" />
     <Compile Include="Graphics\QuadRenderer.cs" />
     <Compile Include="PSubVec.cs" />
     <Compile Include="PSubPos.cs" />
diff --git a/OpenRA.Game/World.cs b/OpenRA.Game/World.cs
index 7c2d6177a6..d1ea2004a0 100644
--- a/OpenRA.Game/World.cs
+++ b/OpenRA.Game/World.cs
@@ -111,6 +111,7 @@ namespace OpenRA
 			}
 		}
 
+		public DomainIndex WorldDomains;
 		internal World(Manifest manifest, Map map, OrderManager orderManager, bool isShellmap)
 		{
 			IsShellmap = isShellmap;
@@ -121,6 +122,9 @@ namespace OpenRA
 			TileSet = Rules.TileSets[Map.Tileset];
 			TileSet.LoadTiles();
 
+			// Identify untraversable regions of the map for faster pathfinding, especially with AI
+			WorldDomains = new DomainIndex(this);
+
 			SharedRandom = new XRandom(orderManager.LobbyInfo.GlobalSettings.RandomSeed);
 
 			WorldActor = CreateActor( "World", new TypeDictionary() );
diff --git a/OpenRA.Mods.RA/Move/PathFinder.cs b/OpenRA.Mods.RA/Move/PathFinder.cs
index 5345bb0702..40d3e84fcd 100755
--- a/OpenRA.Mods.RA/Move/PathFinder.cs
+++ b/OpenRA.Mods.RA/Move/PathFinder.cs
@@ -44,6 +44,9 @@ namespace OpenRA.Mods.RA.Move
 		{
 			using (new PerfSample("Pathfinder"))
 			{
+				// If a water-land transition is required, bail early
+				if (world.WorldDomains.IsCrossDomain(from, target)) return new List<CPos>(0);
+
 				var cached = CachedPaths.FirstOrDefault(p => p.from == from && p.to == target && p.actor == self);
 				if (cached != null)
 				{
@@ -72,6 +75,13 @@ namespace OpenRA.Mods.RA.Move
 		{
 			using (new PerfSample("Pathfinder"))
 			{
+				// As with FindUnitPath, avoid trying to traverse domain transitions.
+				// In this case it's pretty sketchy; path false-negatives are possible.
+				if(world.WorldDomains.IsCrossDomain(src, target.ToCPos()))
+				{
+					return new List<CPos>(0);
+				}
+
 				var mi = self.Info.Traits.Get<MobileInfo>();
 				var targetCell = target.ToCPos();
 				var rangeSquared = range.Range*range.Range;