The interior of the cave is filled with water, and won't be revealed by entering it like most other locations. To see inside the cave, there are currently 6 known methods:
- The player can drink worm blood, which is obtained by killing a blue worm. A wiggling egg to hatch one from is conveniently available from the Giant Tree above. The night vision effect of the worm blood is temporary, so it's recommended to either bring the egg with you to kill the worm near the cave entrance, or use an empty flask to collect the worm blood after killing it so you can take it where it's needed.
- You can also use a wand with the Torch or Electric Torch passive. Wands with the former can sometimes be found in the Mines biome.
- Explosive Bounce modifier can light up the area for a short flash, despite it activating at a range, Immunity Perks are strongly recommended.
- The Black Hole spell lights up a small area while it travels.
- Projectiles with the Light modifier can briefly light up the area, as well as the Lightning Bolt spell by itself.
- Finally, you can return here via the eastern shaft from the Snowy Depths after getting the All-Seeing Eye Perk (not the spell), which will remove the darkness completely.
While a bit dangerous and tricky, it's absolutely possible to navigate this cave without using any light source at all. Just make sure to drain the water first so you won't drown, and pray you won't have any enemies wander in from the west to shoot you in the dark. Good ways of doing this include cutting a hole near the bottom towards an open cave, or bringing a Kiuaskivi and holding it.
This content includes possible spoilers or secrets. Tread carefully.
Below are the 5 currently available Dark Cave layouts. Red for Max HP bonus, yellow for the Full Health Regeneration. 
- In case the game will ever update this feature, the new files will most likely be located under
Nolla_Games_Noita/data/biome_impl/folder, unless the generation is made procedural.