We have barely managed to drill the surface to a depth of about 12 kilometres (to the centre of the Earth is 3,000 kilometres): an artificial hole created in Russia, on the Kola Peninsula, in 1962, whose goal was to reach a very deep layer of the Earth. Thanks to this we also discovered that the water remains in a liquid state at such a depth, or that the temperature increases with depth in a greater proportion than previously thought.
What lies beneath the surface of our planet therefore remains a mystery, as is its extraordinary ecosystem, as biodiverse as that of the Amazon. For example, almost 70% of all microbes on Earth are under our feet. Additionally, underground organisms account for 15 to 23 billion tons of carbon, hundreds of times more than the content in all human beings. Not surprisingly, the subterranean biosphere has between 2,000 and 2,300 million cubic kilometres, almost twice the volume of all oceans.
In spite of everything, until a few centuries ago there was no proof of the existence of life underground.
Johann Weikhard von Valvasor
The first confirmed discovery of underground life was not made until 1689, thanks to the explorations of Karst, a cave-infested region in Slovenia that were carried out by Baron Johann Weikhard von Valvasor, a nobleman of Trieste. The historiography that Valvasor published described a snake-like creature that was about thirty centimetres long and that left the caves when a heavy storm fell.
Valvasor baptized the creature as a olm (Proteus anguinus), and it was a salamander that lived underground, although the locals who had encountered it described it as a branch of some dragon. As Will Hunt explains in his book Underground: "In The Origin of Species, Charles Darwin cited the olm as an example of his theory of adaptive evolution: in his day he was part of a population that inhabited the surface, but began to spend more time in underground environments, perhaps seeking refuge from predators and, gradually, over the course of millions of years, the beneficial physical features of underground life were transmitted".
Due to the new ecosystem in which the olm flourished, it lost its pigment, causing its skin to become white, as it no longer needed protection from ultraviolet rays. His eyes also ceased to see because it did not need vision in such darkness. Likewise, it was able to spend a whole year without food.
Hunt explains that, shortly after, researchers came to identify more examples of creatures inhabiting the caves, which were divided into:
Animals of the shadows: they inhabited the entrance of the caves.
Animals of the twilight: they lived within reach of the diffuse light.
Animals from the dark zone or troglobites: they could not inhabit the surface, like the olm.
Expeditions to the caves revealed a dreamlike fantasy of troglobites: albino catfish, pearly spiders, blind beetles, transparent crabs and insects without eyes.
Little by little, subsequent research revealed the enormous complexity of the underground ecosystem, enlightening us to creatures capable of surviving in environments similar to those of an extra-terrestrial planet, without oxygen, without light, under extreme heat and many other characteristics that, until recently, were thought incompatible with life.