Barotrauma is a 2D online co-op submarine game taking place on Jupiter's moon Europa. The game takes heavy inspiration from a game called Space Station 13. The players join a crew of up to 16 players and try to carry out missions such as salvaging alien artifacts, killing enormous sea creatures and hauling dangerous cargo from outpost to another. The crew won't just have to deal with external threats like the pressure outside the sub or the inhabitants of the ocean, as the submarine itself is a complex contraption with a load of interconnected items and devices which can cause a wide range of non-scripted, dynamic disasters.
The game is currently in alpha stage, although it's already fully playable and has an active and steadily growing fanbase. Full release on Steam is estimated for late 2017, but until then, developers Undertow Games will keep releasing in-development versions of the game for free.