The Hollow Earth is the name of a hypothetical location which many ancient and prehistoric monsters, also known as Titans originated from. Many ancient civilization or natural habitat that a Titan live in have a connection to the Hollow Earth.

History
In the Precambrian Era, Gojira dig into the Earth itself as she many various of tunnels and headed into near the center of the planet, this is where the Hollow Earth was created. In million of years within the Hollow Earth, Gojira created an ecosystem full of plant life thrive on and leaving fragments of her energy all around  which cause gravity to be different. In the beginning of Permian period, Gojira laid her first egg which hatch into the future king of the monster Dagon but is seem more lifeform have evolved which now creatures called Titans have rise within their birth home. Many of the Titans move out as they live on surface but Gojira remain there due to see the place as her true home. In the Titan-Kaiju Wars, the invading Kaiju try to enter into the Hollow Earth for their masters the Precursors as they see it might change the Triassic Period planet into a more suitable home but failed. Many Titans on the surface would return to the Hollow Earth when the Earth's radiation is low, they would pop out sometime but also make new home where they were been worship as gods and/ or guardian of their civilization. Within the centuries, many humans have discovered or heard of the Hollow Earth like Edmond Hailey, John Cleves Symmes Jr., Jeremiah N. Reynolds, and even Jules Vern for his story Journey to the Center of the Earth. In 1954 after the death of MUTO Prime, Monarch search around the Earth in hope to find these monsters as they believe that they come from the depth. In 2000, Monarch bought vehicles called H.E.A.V. in hope to travel into the Hollow Earth but try to find a better way due to tectonic plates shifting

Known Titans & Creatures from the Hollow Earth
Titanus Goji: Gojira, Dagon, Raiden, Servum, etc - Various locations
Titanus Mosura: Mothra, Leo, Battra, Mosura, etc - Infant Island
Titanus Manda: Manda & Mammoth Serpents - Mu
Titanus Gorozaurusu: Gorosaurus & Goros - Mondo Island
Titanus Baragon: Baragon & Baranosdragons - Various locations
Titanus Megaro: Megalon  & Megarons - Seatopia & Mu
Titanus Shisa: King Caesar & Shisas - Okinawa
Titanus Chitanozaurusu: Titanosaurus & Chitanosauruses - Ogasawara Islands
Titanus Meganyura: Megaguirus & Meganulons -  Various locations
Titanus Muto: MUTOs - Various locations
Titanus Mətusalah: Methuselah - Mulch, Germany
Titanus Skylla: Scylla - Sedona, Arizona
Titanus Behemot: Behemoth - Rio, Brazil
Titanus Nozuki: Warbats - Remain in the Hollow Earth
Titanus Yoshima: Hellhawks - Remain in the Hollow Earth
Titanus Orochi: Yamata no Orochi - Izumo, Shimane
Titanus Maguma: Maguma - Antarctica
Titanus Daigoro: Daigoro and his mother - Unknown
Titanus Gogo: Ghogo - Nilai-Kanai Civilization
Titanus Redozaurusu: Rhedosaurus - Arctic
Titanus Repticus: Reptilicus - Lapland
Titanus Gorgon: Gorgo & Ogra - Nara Island
Titanus Yong-gali: Yonggary - Middle East
Titanus Hakaisha: Clover - Liberty Island
Titanus Gehara: Geharha - Unknown
Titanus Shinomura: Shinomura
Titanus Amhuluk: Amhuluk - Manpupuner Rock Formation, Komi Republic
Titanus Na Kika: Na Kika - Indian Ocean
Titanus Tiamat: Tiamat - Stone Mountain, Georgia, US
Titanus Camazotz: Camazotz - Oaxaca, Mexico
Titanus Garashapu: Garasharps - Atlantis
Titanus Baruguzadan: Balkazardan - Unknown
Titanus Jarumu: Jyarumu - Unknown
Titanus Rajin: Razin - Unknown
Titanus Vagunozaurusu: Vagnosaurus - Unknown
Titanus Faiaraion: Fire Lion - Mu
Titanus Vishnu: Rokmutul - Janjira