Undertow Cavern is a large sea cavern and one of the caverns in The 99 Caverns. It's known for its vast sea landscape making it the preferred habitat of most water type slugs and species, but it's also known for being hosted by pirates.
- Undertow (First appearance)
- Deep Water, Dark Water
- Dark as Night (Mentioned only)
- Slugterra: Return of the Elementals
- Undertow is home to the Mimkey, Aquabeek, and all other Water Slugs.
- Undertow cavern can be entered through The Flumes or a single land passage.
- One common predator is the Sharks, an animal that preys on slugs. They may also eat humans and Amphibanoids, though the sharks may just think of them as hostile beings and are just defending their territory.
- This was formerly the home of the Water Elemental until it was hunted by The Goon and Dr. Blakk.