Entering Diyu.

A mining expedition in the Yunnan province of China accidentally unearths a hidden cave via blasting undiscovered by mankind. A group of miners ventures into the cave and they are astonished by the extent of it; the cavern leads deeper and deeper and after half an hour of walking there is still no end in sight with the tunnel eventually leading them to a sheer cliff face plummeting into nothingness. One of the members slips and plummets into the abyss and a rescue attempt is quickly assembled; they travel deep into the darkness to locate him. After 5 hours they discover him limping through the blackness, injured but still alive; however, his mind has been broken by unseen things and he speaks of horrible monsters and demons and places humanity was not meant to go. The next day, a further expedition was made to see what he saw.And what they saw was perhaps something mankind was truly not worthy of seeing.------Almost a kilometre beneath the surface of the Earth lies a cave system completely untouched for countless millennia. Carved out by natural processes for millions of years, it now stretches for dozens of miles and at its highest is over 225 metres from the ceiling to the ground. To this day no one is really sure how large it truly is and it's believed the entirety of it has yet to be completely mapped. Indeed, the cave is large enough to hold within it a small lake and a system of waterways running through many of its tunnels and is easily one of the most impressive caverns on Earth.But perhaps a first for any cave is that the grand features of this place perhaps pale in comparison to the incredibleness of the native life-forms. While many caves have a spectacular and bizarre collection of lifeforms, they are no comparison to the awe-inspiring flora and fauna found within Diyu.The caves seem to have been completely isolated from the outside world for over half a billion years and life found within the caves seems to be almost entirely descended directly from Early Cambrian species. Many extant species found within the caverns indeed bare some resemblance to fossilized species from the nearby Maotianshan Shales, and the sparse fossils known from Diyu are almost identical to those found in the neighbouring formation (it should be noted that fossils within the cave system are actually abundant, but the delicate nature of the caverns' ecosystems and the difficulty of which to navigate and unearth said fossils, as well as the bias towards studying living animals means few are excavated). It is apparent that sometime 515 million years ago some organisms became isolated deep underground and undisturbed by mass extinctions and the changing world around them they developed on separate evolutionary paths without any influence from outside factors.Hundreds upon hundreds of species are known from Diyu, and it is believed to have an endemism rate of 100%, with no species known that are descended from organisms later than the Early Paleozoic. In short, every species from animals, fungi, protists, bacteria, archaea, and the rare plants known from Diyu are completely unique to this world beneath.---The ecosystems within Diyu are almost unbelievably diverse considering it operates entirely without influence from the Sun. The basis of its food-chain instead rests upon lithotrophic and chemotrophic bacteria and archaea as well as a huge variety of fungi species; plant species are rare but a few algae-descended species manage to survive via symbiotic relationships with mycorrhizal fungi and sometimes limited photosynthesis based upon bioluminescence (organisms that glow are common within Diyu). From there are animals that graze upon these producers and then a huge array of predatory animals that have evolved to prey upon them. Although the ancestors of these organisms were all presumably marine, the wet and humid environment has since allowed many to adapt to land.The sheer size of the caverns running beneath the Earth's surface means a spectacular array of life-forms have adapted to many specialized niches; when explorers first entered Diyu they were awed by the otherworldly and alien nature of the flora and fauna, and perhaps imagined they had been somehow transported to another planet. Over half a billion years of evolution completely isolated from the surface has created organisms bizarre beyond imagination, strange and incredible creatures truly like nothing else on Earth.---