The Catacombs is a room in the subterranean level of Torvus Bog. It initially is accessed from the Training Chamber via Transit Tunnel East and can be accessed from Catacombs Access via a Purple Hatch once the lock in the Hydrodynamo Station has been undone, as well as the Transit Tunnel South using a Grey Hatch.
On your first visit, there will be two Bloggs in the water swimming around, which are replaced with Grenchlers upon visiting later. There are also several Torvus Bearerpods loitered around the room, mostly on the upper 'land' parts of the room. The body of one of the Luminoth Keybearers named G-Sch can be found in this room along the eastern wall, next to the exit to the Catacombs Access. Scanning him reveals he died fighting against the Ing and their leader, who he calls "a foe worthy of a Luminoth Warrior."
After defeating the Alpha Blogg in the Main Hydrochamber and getting the Gravity Boost from the Hydrochamber Storage room, Samus can use a Bomb Slot hidden under the water to access a Dark Portal leading to the Dungeon in Dark Aether. This is required for players to get the Grapple Beam and progress through Torvus Bog.