From the introduction: Lava Flows add cross-tile magma to Dungeons in Blue geomorphs so you can map out the villain's volcanic lair or a dungeon on the plane of fire. Much like cross-tile water features, lava rivers and lakes connect but won't match up with non-lava tiles. Most of these tiles include at least one edge with no lava connections so they can be used with regular geomorphs. The set has two versions of each tile, one for players, and one for GMs. The GM's version shows additional features, tricks, and traps.