Stoick: "Legends tell of ships that sail too close to it, only to drop off the edge of the world, never to be seen again. But those sailers who turn back tell tales of a great waterfall and dragons guarding the entrance to a hidden world."
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.
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 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 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.
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.
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 the a large part of the Hidden World.
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.
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 took billions and billions of years to grow just half of the size of the ones that are shown in the film.