Hidden World

The Hidden World is the major and titular location that is featured in How to Train Your Dragon: The Hidden World.

Official Description
"The Hidden World is an old sea-faring legend about a land at the edge of the world, where many sailors have lost their ships — never to be seen again. Those who have returned reported sightings of a great waterfall and dragons guarding the entrance to a hidden world beneath the sea. A land from which all dragons hail."

- Dean DeBlois

"This island still holds too many secrets."

- Dragons: Rise of Berk

Development
"We wanted something that transcended the idea of a cave. Part of that was delving into bioluminescence and was of channeling light into this subterranean space with the use of crystals that could carry light from magma. We wanted it to be an exotic world that still feels like it belongs here on earth. But it's still a place for dragons and not for humans."

- Dean DeBlois

Geography
Dean DeBlois confirmed that the Hidden World is located within a massive volcanic caldera that serves as a gateway to an endless maze of chambers and tunnels, which in turn work their way around the world.

This subterranean world contains bioluminescent fungi, as well as coral and bright crystals.

The Caldera
"The Caldera is a giant collapsed volcano in the middle of the ocean, which serves as the entrance to the Hidden World. The only way to enter the Caldera is by flying in on a dragon."

- The Art of How to Train Your Dragon: The Hidden World

Mushroom Forest
"After a dragon has entered the Hidden World through the Caldera they get acccess to a network of caves deep in the crust of the earth. After a dragon flies through this network they reach the Mushroom Forest, where the Hidden World's bioluminescence is revealed. The Mushroom Forest mainly gives home to many stalactites and stalagmites that are completely overgrown with various sorts of mushrooms."

- The Art of How to Train Your Dragon: The Hidden World

King Island
"At the center point of the Hidden World lies King Island, which serves as the throne room for the king of dragons. The throne for the Alpha is a gigantic crystal on the island, looking over a large part of the Hidden World."

- The Art of How to Train Your Dragon: The Hidden World

Hidden World Annex
"In the darkest depths lies the unknown and it is up to Vikings to explore it! Very few Vikings have been valiant enough to venture into its depths, but those few who have get to witness the light and bioluminescent found within. As dragons seem to take a shine to these underground caves and brightly glow throughout them."

- School of Dragons

How to Train Your Dragon: The Hidden World
The Hidden World is a location that is heavily featured in the third film. Hiccup and Astrid discovered it while searching for Toothless on Stormfly before being chased out by the dragons that reside there.

Dragons: Rise of Berk
The Hidden World first appeared as a special searchable location in December 2018.

Trivia

 * The Hidden World is a famous myth/legend from the distant seas. It is sometimes called "a world beyond the sunset".
 * The Hidden World appears to be an incredibly ancient location due to how massive the stalagmites and stalactites are. This is because rock formations take billions and billions of years to grow just half of the size of the ones that are shown in the film.
 * Dean DeBlois first imagined the Hidden World in a dream as a hole in the sea.
 * Hiccup and Astrid are the only known Vikings to have ventured into and out of the Hidden World.
 * According to Simon Otto, more than 65,000 dragons reside in the Hidden World.
 * According to Brad Lewis, New Berk is supposed to be the mirror image of the Hidden World.