Hollow Earth

The Hollow Earth, known to the Iwi as the Long Ago Below, is a location that was discovered in the 2019 film Godzilla: King of the Monsters.

It is an ancient subterranean ecosystem deep within the Earth, close to the planet's core. Its gravity is unique and inverts at a halfway point, allowing Titans to walk freely on both floor and ceiling of the Hollow Earth without fear of falling from one to the other. It's surrounded by a gravitational-electrostatic "membrane" anomaly inside the Earth's mantle. The ecosystem continually radiates radioactive energy, allowing the creatures that inhabit it to evolve to titanic sizes. A fortress belonging to Kong's species also exists there, with a throne once held by his ancestors.

Backstory
The Hollow Earth is believed to be the natural habitat and evolutionary place of origin of all Titans. In ancient times, Kong's species warred against Godzilla and/or a member of his species for dominance within the Hollow Earth. Iwi legends suggest that the war ended with Titanus Gojira's victory and with the Kongs, and the Iwi who'd originally lived alongside them in the Hollow Earth, being banished to Skull Island on the surface world.

Godzilla: King of the Monsters
After nearly being killed by the Oxygen Destroyer, Godzilla retreats to his temple located within the Hollow Earth to rest and regenerate. A Monarch expedition team follows the Titan in, confirming the organization's theories of an entire world located beneath the surface. Ishiro Serizawa detonates a nuclear warhead in Godzilla's temple, effectively reviving him but destroying the temple and the surrounding area.

Godzilla vs. Kong
A joint expedition team of Monarch and Apex Cybernetics use three HEAVs to follow Kong into the Hollow Earth in order to find and secure an energy source capable of subduing Godzilla, using the ape as a sort of navigator. Those who have been watching over Kong at Skull Island, including Nathan Lind, Ilene Andrews and Jia, are brought along on the expedition, but the true purpose of the journey is hidden from them.

Kong explores Hollow Earth at a rapid pace before stumbling across an ancient fortress built by his ancestors. He enters the structure, claiming an ancient axe nearby a unidentified titan's corpse. The group observes Kong and realizes that he is home there. At this point, Maia Simmons of Apex orders the extraction of the ancient energy radiating from the ground, directly from the planet's core. After angering Kong, Simmons and the rest of her team are killed, but not before the energy is sent back to the Apex facility in Hong Kong and used to power Mechagodzilla.

After destroying Mechagodzilla and resolving his now ending rivalry with Godzilla, Kong decides to return to the Hollow Earth, making it his new home.

Local flora and fauna

 * Godzilla (momentarily)
 * Kong
 * Ancient Titanus Gojiras (deceased)
 * Ancient Titanus Kongs (deceased)
 * Unidentified Titanus Kong individual (deceased)
 * Warbats
 * Hellhawks
 * Red Leafwings
 * Rock Critters
 * Hollow Earth lizards
 * Genitor Sub-Titan (deceased)
 * Genitor swarm (deceased)
 * Camazotz
 * Camazotz's Minions
 * Skullcrawlers (presumably)
 * Shimidah (cut)
 * Tiamat (momentarily)

Geographical Features

 * The Mountain Hill
 * Mainlands
 * Kong's temple (destroyed)
 * Entrance to Hong Kong
 * Godzilla's lair
 * Hollow Earth waterfall