Over 90 percent of caves are formed in a solutional rock, usually limestone. To explore you need to find areas with limestone beds. There are numerous areas like this worldwide. They're all unique because of how the caves are formed.
Limestone is a sedimentary rock, meaning it formed by particles in a shallow tropical ocean billions of years ago. So what fell to the sea bottom varies. There are strong and weak portions of the rock. The bits that dissolve leaving behind tunnels are down to the lay of the rock.
Millions of years ago, the seas were filled with very different kinds of life forms than today. Over the millennia, soft seafloor turned into limestone, preserving a snapshot of these creatures from the past. Caves cut right through these ancient layers, displaying a rich collection of fossils and telling a geological story of the Earth’s past. Taking a close look, a bulge in the cave ceiling can turn out to be a tooth of an extinct tiger.
The characteristics of a cave can completely change over a distance of 100 meters, one reason making cave diving so enticing. Think about swimming through 100 metres of cave passage today, and memorise all of it. And the getting into a time machine and go 3,000 years into the future. The cave would be different because you're diving in something that is alive. The acidic rainwater is dissolving rock, and the flow of the water is eroding it. So the cave is growing. It's forming. It's changing as you're swimming through it.
You're not only exploring, but you're also seeing something that's alive and changing like a tree would be bigger if you came back in ten years' time.There can be things that are equally capable of living in the outside world, like fish and small crustaceans like little mini-lobsters.
There's a unique little creature called a Proteus that's evolved to live permanently in a cave, so they've lost their pigmentation or don't have eyes. It just lives in the caves of northern Italy, Croatia, Slovenia and Serbia. And deep into very rare cave systems, you can find a live system evolved around chemosynthesis, much like the black smokers in the ocean vents in a deep ocean. A tiny little ecosystem that evolved from them would continue to survive even if the sun went out. Because they don't use photosynthesis in any part of their ecosystem.
You can't tell if new forms of life can be found because it's like the bottom of the ocean. We don't know because we haven't been to every part of the bottom of the ocean. So, it would be a pure hypothesis to guess that there's something down there that we've not seen. There's no Loch Ness monster because there's no food. Do the maths. So it's going to be slime. If there's something unknown, it's going to be slime, it's going to be bacteria or something tiny that you'd identify in a petri dish rather than jumping out from behind a rock and going he-he.