Who knows what wonders may lie beneath our feet. Herds of prehistoric creatures in deep underground caves were described in the Jules Verne novel Journey to the Center of the Earth. And when 17th-century scientists discovered the first troglobite - a creature that lives permanently in a cave - they initially thought it was the offspring of a dragon.

Science, of course, has a habit of turning the fantastic into the prosaic.But 150 years after Verne's work, researchers began a project to drill through the Earth's crust for the first time, hoping to penetrate more than 3 miles below the seabed to reach the mantle below.Needless to say, it's highly unlikely to reveal monsters living inside the Earth.But if we look atdown in search of life, what do we find?

Journey to the center of the Earth

The best way to find underground creatures is to travel deep into a cave. The first things you're likely to find are spiders - big ones, like Meta menardi (the European Cave Spider.) Meta is actually a troglophile, a species that can spend its entire life underground, but can also live outside of caves. Living up to at least 30 meters inside caves, it makes webs to catch flies, but mostly trapped in woodlice and millipedes on cave walls. Look up and you can see its egg-shaped cocoonsof tears hanging from the ceiling.

The Meta menardi spider is one of the first creatures you might encounter if you're going on a journey to the center of the Earth. (Magne Flåten / Wikimedia , CC BY-SA)

Cave spiders are often very large. They grow slowly, live for a long time and need to store fat from their infrequent meals. Meta is large but dwarfs a species of spider in a cave in Laos in 2001. Heteropoda maxima the giant hunter spider, has a leg span of 30cm and is the largest spider in the world by diameter.

You may also encounter bats. These are trogloxenos, animals that live in caves but gather food from outside. Some species, like the bat scout, spend the entire winter underground, but usually don't hibernate more than a few hundred feet inside.

Leptodirus hochenwartii Yerpo/Wikimedia, CC BY-SA

In complete darkness

Beyond the deep threshold of a cave, it is completely dark. No sunlight means there are no plants to produce food and therefore no herbivores. Instead, cave communities rely on bacteria and fungi that break down any organic matter that dies or is washed into the cave. This is then consumed by legions of tiny mites, crustaceans and insects -referred to as detritivores - which, in turn, are eaten by predators.

These are common in almost all habitats, but in caves they become the predominant mechanism for recycling dead material since it is completely dark. The deeper the subsoil, the fewer food resources are available and therefore the fewer species can survive. The exception is bacteria which appear completely independent of terrestrial energy sources,thriving on minerals dripping from cave walls.

Unable to see prey or predators, troglobites are often eyeless and rely on other receptors, including a highly developed sense of touch. They begin to appear as soon as there is total darkness and become increasingly adapted as you go deeper. Specialized cave beetles, such as the Leptodirus hochenwartii , have developed extraordinarily long antennae and elongated legs for this purpose. Being eyeless will save energy, and Leptodirus saves more by growing directly from a larva to an adult beetle, whereas its more distant relatives have three larval stages.

Mexican tetra, is a depigmented fish that you might encounter further along on your journey to the center of the Earth. Ltshears/Wikimedia

In some North American caves, you may find Mexican tetras ( Astyanax mexicanus or blind cave fish) or the Texas blind salamander ( Eurycea rathbuni) . some Astyanax have eyes and some do not and the two can interbreed, providing valuable evidence that species can vary and evolve hundreds of meters underground. Eurycea is very similar to the dragon-like Proteus, but the species are only distinctly related, giving us a good example of how different animals can evolve similar characteristics in parallel.

Astyanax and Eurycea use pressure sensors to detect food and Proteus has the most sensitive sense of smell of any amphibian. It also uses electric fields to locate prey and can survive for ten years without food. After 16 years, Proteus becomes sexually mature in a juvenile form and can live to over a hundred years. Because there is little energy in a deep cave, thetroglobites often take a long time to mature, so they have longer generation times than their surface-dwelling relatives. With limited resources for reproduction, they tend to lay fewer eggs, but often spend more time protecting them.

Geophilus hadesi (Daniel Mietchan/Wikimedia, CC BY)

Deeper still

Go deeper, and at 1,000 meters deep you can still find life. Most of the inhabitants at this level are small invertebrates, such as whitetails and mites. But there are also larger species, such as Geophilus hadesi Found in Croatia at a depth of 1,100 meters, this eyeless centipede is named after the Greek god of the underworld.

The deepest land animal found in a cave is a wingless, eyeless insect. Plutomurus ortobalaganensis was discovered living at a depth of 1,980 meters in what is also the world's deepest known cave, Krubera-Voronja in Russia.

Plutomurus ortobalaganensis is the deepest land animal ever found on Earth, but it might not be the last living thing you could encounter on the journey to the center of the Earth. (Enrique Baquero / Wikimedia , CC BY-SA)

Many cave systems are isolated from the rest of the world like islands in an ocean and, as a result, are very fragile habitats. The creatures that live in them tend to become genetically very similar to the rest of their species thanks to inbreeding, putting them at risk of genetic deformities and even extinction if their environment changes even slightly.

When 18th and 19th century navigators explored unknown islands, they often found new species evolving as a result of millennia of isolation, and the same is true of caves. Each new cave has the potential to harbor unique species and teach us more about how life can survive in extreme habitats. There is still much more to find in the dark.

By Christopher Terrell Nield.

This article was originally published by The Conversation and republished under license, read the original article by clicking here.