The Ocean Trench Key open Ocean Trench dungeon.
The Ocean Trench is a high-level dungeon with an underwater theme. It is a primary source of Potions of Mana, as well as dropping a UT Huntress set.
The most important difficulty factor is that players need oxygen. An oxygen counter gradually goes down until the screen flashes red, which is when players start losing health very quickly (faster than when you stand in magma or lava, almost -100 hp/sec). To refill oxygen, players must stand on top of bubble vents that refill the oxygen counter. If an oxygen source is not found and your hp reaches 0, you die from “Damage Suffocation”.
The entrance to the Ocean Trench is dropped from the Hermit God at a guaranteed rate.