Dark Cave

The Dark Cave is located west of the Collapsed Mines, and connects to the Ancient Laboratory further west.

About
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 5 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 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.

Once the player can see inside the cave and the water is removed (by drinking or other means), a Max health upgrade pickup can be found in the top half. A full heal pickup (much like the one in the Holy Mountain) can also be found near the bottom, making this one of the few sources of healing.

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, a good way of doing this is blowing a hole near the bottom towards an open cave or bringing a Kiuaskivi and holding it.

Possible layouts
Below are the 5 currently available Dark Cave layouts. In case the game will ever update this feature, the new files will most likely be located under  folder, unless the generation is made procedural.