Pfhlow is a multiplayer carnage and king of the hill level that uses some in terms of the liquids. It is highly recommended that you download and install the Texture Enhancement pack, or get a script for transparent liquids. This level comes with its own built-in physics and five variances of the map (different in more than just their liquids), all available in this package.
This map is rather large with many rooms. There's much to explore, containing many secrets along the way. The map can be played solo, there are plenty of aliens throughout the level. It's the same with multiplayer co-op. If you are playing every man for himself, with more than two players, I suggest turning the aliens off. Have fun!