Index: src/terraform_cmd.cpp
===================================================================
--- src/terraform_cmd.cpp	(revision 16237)
+++ src/terraform_cmd.cpp	(working copy)
@@ -303,9 +303,10 @@
 				return_cmd_error(STR_ERROR_MUST_DEMOLISH_BRIDGE_FIRST);
 			}
 			/* Check if tunnel would take damage */
-			if (direction == -1 && IsTunnelInWay(tile, z_min * TILE_HEIGHT)) {
+			if (IsTunnelInWay(tile, z_min * TILE_HEIGHT)) {
 				_terraform_err_tile = tile; // highlight the tile above the tunnel
-				return_cmd_error(STR_ERROR_EXCAVATION_WOULD_DAMAGE);
+				if (direction == -1) return_cmd_error(STR_ERROR_EXCAVATION_WOULD_DAMAGE);
+				if (direction == 1) return_cmd_error(STR_ERROR_MUST_DEMOLISH_TUNNEL_FIRST);
 			}
 			/* Check tiletype-specific things, and add extra-cost */
 			const bool curr_gen = _generating_world;
Index: src/tunnel_map.cpp
===================================================================
--- src/tunnel_map.cpp	(revision 16237)
+++ src/tunnel_map.cpp	(working copy)
@@ -41,18 +41,14 @@
 bool IsTunnelInWayDir(TileIndex tile, uint z, DiagDirection dir)
 {
 	TileIndexDiff delta = TileOffsByDiagDir(dir);
-	uint height;
-
-	do {
+	for (;;) {
 		tile -= delta;
 		if (!IsValidTile(tile)) return false;
-		height = GetTileZ(tile);
-	} while (z < height);
-
-	return
-		z == height &&
-		IsTunnelTile(tile) &&
-		GetTunnelBridgeDirection(tile) == dir;
+		if (GetTileZ(tile) != z) continue;
+		if (!IsTunnelTile(tile)) continue;
+		if(dir == GetTunnelBridgeDirection(tile)) return true;
+		return false;
+	}
 }
 
 /**
Index: src/tunnelbridge_cmd.cpp
===================================================================
--- src/tunnelbridge_cmd.cpp	(revision 16237)
+++ src/tunnelbridge_cmd.cpp	(working copy)
@@ -504,7 +504,9 @@
 	} else {
 		tunnel_in_way_dir = (TileY(start_tile) < (MapMaxX() / 2)) ? DIAGDIR_SE : DIAGDIR_NW;
 	}
-
+	if (!_cheats.crossing_tunnels.value && IsTunnelInWayDir(start_tile, start_z, tunnel_in_way_dir)) {
+		return_cmd_error(STR_ERROR_ANOTHER_TUNNEL_IN_THE_WAY);
+	}
 	end_tile = start_tile;
 
 	/** Tile shift coeficient. Will decrease for very long tunnels to avoid exponential growth of price*/
@@ -513,18 +515,33 @@
 	int tiles = 0;
 	/** Number of tiles at which the cost increase coefficient per tile is halved */
 	int tiles_bump = 25;
+	/** Tunnel under water flag */
+	bool passing_water = false;
 
 	for (;;) {
 		end_tile += delta;
 		if (!IsValidTile(end_tile)) return_cmd_error(STR_TUNNEL_THROUGH_MAP_BORDER);
 		end_tileh = GetTileSlope(end_tile, &end_z);
 
-		if (start_z == end_z) break;
+		if (start_z == end_z && IsTunnelTile(end_tile)) return_cmd_error(STR_ERROR_ANOTHER_TUNNEL_IN_THE_WAY);
 
+		if (end_z > start_z || end_tileh == InclinedSlope(ReverseDiagDir(direction))) passing_water = false;
+
+		if (IsTileType(end_tile + delta, MP_WATER) && !IsWaterTile(start_tile - delta) && end_z == 0) {
+			if (GetWaterTileType(end_tile + delta) == WATER_TILE_CLEAR ||
+				GetWaterTileType(end_tile + delta) == WATER_TILE_COAST) passing_water = true;
+		}
+
 		if (!_cheats.crossing_tunnels.value && IsTunnelInWayDir(end_tile, start_z, tunnel_in_way_dir)) {
 			return_cmd_error(STR_ERROR_ANOTHER_TUNNEL_IN_THE_WAY);
 		}
 
+		if (start_z == end_z && !passing_water) {
+			/* Don't allow tunnel inbetween existing tunnel */
+			if (IsTunnelInWayDir(end_tile, start_z, direction)) return_cmd_error(STR_ERROR_ANOTHER_TUNNEL_IN_THE_WAY);
+			break;
+		}
+
 		tiles++;
 		if (tiles == tiles_bump) {
 			tiles_coef++;
Index: src/water_cmd.cpp
===================================================================
--- src/water_cmd.cpp	(revision 16237)
+++ src/water_cmd.cpp	(working copy)
@@ -355,6 +355,9 @@
 				return_cmd_error(STR_ERROR_TOO_CLOSE_TO_EDGE_OF_MAP);
 			}
 
+			/* Don't allow water to be cleared above underwater tunnels */
+			if (IsTunnelInWay(tile, 0)) return_cmd_error(STR_ERROR_ANOTHER_TUNNEL_IN_THE_WAY);
+
 			/* Make sure no vehicle is on the tile */
 			if (!EnsureNoVehicleOnGround(tile)) return CMD_ERROR;