https://youtu.be/vvev4R2PVJQ

This world is called Hollow Earth is another underground world. Not to mention it had a lot of flora and fauna like tribes, animals, plants and fungi. Which is normally what we see in island movies and stories sometimes, well anyways: let's talk about the origins of Hollow Earth.

Millions of years ago, when life started, this underground world had somehow developed underground. Which has hydrothermal vents keeping it warm. Nothing lived on land at that time. But however in the ocean was a different story. In the Cambrian period, early fish and arthropods managed to find their way underground.

Jawless fish colonize every body of water, but unlike their jawed counterparts (which will later evolve) they never get outlasted. Dinocarids and Trilobites managed to colonize the waters too, but somehow they managed to find their way into freshwater and brackish water habitats.

In the Ordovician period, the mollusks have appeared. Which were the primitive orthocones and ammonites, since they were the main ones. There were also the sea scorpions. Despite this, they've also found their ways into brackish waters and freshwater. The fresh plants on land start growing in this warm climate.

In the Silurian period, jawed fish started to appear. Which more invertebrates came as well. So did the other organisms. They've lived alongside their jawless counterparts, but unlike their surface dwelling counterparts however, they didn't outcompete the jawless fish.

In the Devonian period, the early, primitive amphibians started to appear. Along with giant fish like the dunkleosteus, started making their ways into this underground world. The amphibians managed to colonize the freshwater and brackish water ecosystems, but somehow managed to lay claim to arboreal lifestyles in the trees and the ocean here.

Which will be even before the reptiles, then later their mammal and avian counterparts come along. And as for this, since some of them somehow managed to take to the skies since it will be until the meganeura, pterosaurs, birds and bats.

In the Carboniferous period, giant bugs and the early reptiles started appearing. Which the bugs like from the flying meganeura to the 3 meter long arthropleura, migrated here. The amphibians followed as well.

The bugs managed to make themselves at home in the jungles, open woodlands, rainforests, forests, grasslands, tropical forests, arctic regions, rocky mountains and even the deserts here.

As for the reptiles, they normally made the niches like their modern day counterparts, but however: they oddly started filling laying claim to aquatic lifestyles as well, along with arboreal and flying ones too.

In the Permian period: where the synapsids (mammal-like reptiles) started to appear. Which they mostly managed to seek refuge to the underground world as there was a land bridge as well and fill in the niches of mostly their mammalian counterparts that will later come along.

As in many of the lifestyles they somehow lay claim to. Despite this, they also tend to be more strange than their mammalian counterparts. Such as the fact they developed into many shapes and sizes.

In the Triassic period: where the early dinosaurs, pterosaurs and marine reptiles evolved: they found their way into the underground world as well. Which meant the pterosaurs managed to fill in the niches as well.

In the Jurassic period, the giant sauropods found their way down here. Which they migrated here, but got trapped by the storms here. The other types of dinosaurs made it here as well. They began to make themselves at home as well.

In the Cretaceous however, the last dinosaurs, pterosaurs and other fauna in the Mesozoic era got trapped here. The entrance got blocked as the KT event wiped out most of life on earth. The survivors here manage to have the hydrothermal vents keep them alive, as their other neighbors died out elsewhere.

In this time period however, mammals, giant birds, giant reptiles and giant fish were raised. The mammals made it here. At first most of them: even the carnivores such as the creodonts use this as a place to migrate here. Since the dinosaurs and synapsids live here. But due to many changing events, they colonized here.

The birds and reptiles too as well. Even the early humans migrated here as well. The land bridge tripped them here and the entrance was closed. So Hollow Earth continued in peace. The hydrothermal vents were keeping it and it's inhabitants warm. It was never discovered throughout the age of men. Even the greatest explorers never found it. Which will all change for a recent thing for the time being.