Caliban Caves (Books)

The Caliban Caves is a cave system underneath the Isle of Berk.

History
As told in How to Be a Pirate and How to Break a Dragon's Heart, after the fall of the Kingdom of the Wilderwest, King Grimbeard the Ghastly buried a tiny amount of his treasure in a chest on the Isle of the Skullions, and the rest of his treasure underneath the Caliban Caves. Grimbeard placed a baby Monstrous Strangulator here intending for him to grow into a fearsome guardian of the treasure. Grimbeard intended for his true heir to find the treasure as well as his favorite sword.

Around a hundred years, Hiccup Horrendous Haddock III, Fishlegs No-Name, Toothless, and Alvin the Treacherous arrived here after sinking to the bottom of the sea during the Battle of the Lucky Thirteen. Toothless sniffed out the true treasure and a swordfight between Hiccup and Alvin broke out. The Monstrous Strangulator showed up and devoured Alvin, but before he could devour Hiccup, he tricked the Monstrous Strangulator into killing himself. Hiccup, Fishlegs, and Toothless then escaped the Caliban Caves using Toothless' underwater breathing.

Alvin then turned out to be alive and cut himself out of the Monstrous Strangulators stomach with the Stormblade. Having no dragons to escape the Caliban Caves, Alvin was forced to walk through them for months to get to the surface of Berk.