When introducing a giant monster, it's always easiest to explain them away with aliens, extradimensional portals, or magical enchantment. It's a little tougher to explain where a towering beast might have been hiding for all of recorded history. The unforgiving depths of the sea and hostile recesses of the jungle have been popular options, but the Monsterverse has gone above and beyond by hollowing out the planet.
The Monsterverse is the latest attempt to combine the worlds of multiple beloved kaiju. Godzilla and King Kong have been intertwined since the early days, but their new iteration has been built from the ground up to fit together. It's a solid cinematic universe connected by a few fascinating sci-fi concepts. Most notably, Godzilla, King Kong, and all their allies and enemies are from the same place now.
What is the Hollow Earth?
A massive tropical ecosystem sits between the Earth's mantle and core. Gravity functions differently within the Hollow Earth, consistently inverting at certain points in the planet's landscape. Some scientists speculate that the Hollow Earth is the origin point for all life on the planet. Legends about beings coming out of the ground come from the planet's mysterious core. There are a few natural areas through which beings in the Hollow Earth can surface. Some of the deepest areas of the ocean offer holes into the Hollow Earth. Skull Island seems to be an all-purpose home for a variety of creatures from the underworld. Every organism within its expanse is consistently bombarded with radiation. As a result, the Hollow Earth is home to a wide variety of strange and dangerous beasts.
What lives in the Hollow Earth?
The Hollow Earth is the ancestral native habitat of all kaiju species. The only humans who ever dwelled below the surface are the Iwi, who have long since moved to the surface. Both Titanus Gojira and Titanus Kong originally hail from the planet's mantle. Godzilla's species evolved during the Permian period almost 300 million years ago. They were gradually exterminated by a parasitic insect species, driving them close to extinction. They engaged Kong's species in a war for control over the Hollow Earth. The Gojira were successful, eventually driving all the Kongs to Skull Island. Kong's species once ruled over the world below. Their escape brought many other radioactive beings to the surface. This act populated the island with the Skullcrawlers and Leafwings that can be seen in Kong: Skull Island. Godzilla also used a Hollow Earth temple at the bottom of the ocean to recover from a near-fatal wound in Godzilla: King of the Monsters. The first human glimpse into the Hollow Earth came in Godzilla vs. Kong, in which mankind discovered beasts like the Warbats and Hellhawks.
The comics reveal several more monsters who live beneath the planet's surface. A nightmarish bat monster inspired by a Mayan god called Camazotz came from Hollow Earth. With his sonic screech and army of smaller bat monsters, Camazotz followed King Ghidorah's call to attack the surface world. After Ghidorah fell, Camazotz attacked Kong on Skull Island, retreating to the Hollow Earth after his defeat. There's a massive fish called Genitor who commands an army of piranhas. He once attacked Godzilla in his temple, whereupon the King of the Monsters tore him in half and burned his body. There's even an aquatic dragon called Tiamat, after the Mesopotamian deity. She similarly attempted to ambush Godzilla in his home, but she was able to survive her defeat. It seems that almost every kaiju in the franchise will have some connection to the Hollow Earth.
Is the Hollow Earth based on anything?
The idea that the Earth is hollow and contains multitudes within was first floated in the late 17th century. Astronomer and mathematician Edmund Halley imagined three layers of landscape, each with their own atmosphere, magnetic poles, and rotational speed. The idea was still taken seriously until the mid-1800s, but it's been comfortably recognized as pseudoscience since. Hollow Earth theory occasionally pops up as a detail of various adventure stories and conspiracy theories. That conspiracy theory angle recurs in the Monsterverse franchise, in which every character who talks about it is seen as a crackpot. The canceled 1956 film Bride of Godzilla? would've introduced the Hollow Earth concept to the franchise much earlier, but it's a central fiction of the story now.
The Monsterverse is the eventual evolution of decades of kaiju films. It adopted aspects of the cinematic universe format it helped to inspire, the 90s disaster movie, and more modern soft science fiction. There are a lot of strange elements of the Monsterverse, but the idea of hiding away all of these mighty monsters miles beneath the surface of the globe is a strong pitch. The Hollow Earth is a thematically strong setting and a great explanation for the various kaiju who storm the world above.