Scientists have discovered life deep underground, uncovering not only the answer but also some bizarre creatures in the process.

Our planet is home to a vast array of diverse living organisms that inhabit the land, seas, and skies. However, there remains one biome that, due to its complexity, conceals numerous secrets. There's a good chance that we know almost nothing about it.

In a new study published in Science Advances, researchers seek to answer the question of how deep life can exist beneath the Earth's surface. And spoiler alert — we’re talking about several kilometers.

To investigate this, scientists from the United States and Germany examined approximately 50 locations worldwide, delving into the ground, the depths of the ocean, and even hydrothermal vents.

They successfully discovered biomes at a depth of 491 meters beneath the ocean floor and at a depth of 4,375 meters underground on land. Of course, this refers to unicellular organisms — bacteria and archaea, including some that we've never heard of before.

Surface samples were collected from soils, sediments, or water columns, while subsurface samples were extracted from wells, mines, aquifers, or fluids obtained from hydraulic fracturing.

It is generally believed that the deeper one goes, the less energy is available for living organisms, which should consequently result in lower biodiversity. However, in practice, scientists found that in some deep ecosystems, the number of strains could rival those in surface ecosystems. This observation, however, was limited to underwater ecosystems.

Such deep ecosystems lack access to sunlight, experience immense pressure, have virtually no food sources, and are severely spatially constrained. In some areas, microorganisms are forced to inhabit pores in minerals or water that has been trapped between rock layers for millions of years. Extreme conditions demand extreme measures for survival.

The primary energy source there is the breakdown of water due to radiation from the surrounding rocks. The components obtained (oxygen and hydrogen) are utilized by microbes either for methane production or for oxidizing sulfur compounds. Clearly not a buffet, but scientists discovered bacteria at a depth of 3 km that had been living in isolated water for over a billion years. This means they were there about 400 million years before multicellular life appeared on our planet.

Some populations live at such a slow pace that they reproduce only once every 100-1,000 years, and in extreme cases, even once every 10,000 years. In comparison, surface bacteria divide approximately every 20 minutes. Such a slow generational turnover and extremely low metabolism have likely allowed scientists to find some of the oldest existing organisms.

Moreover, the absence of evolutionary pressure has resulted in some populations remaining unchanged for 200 million years. This means since the emergence of the first dinosaurs.

As previously reported, scientists have explored the depths of Lake Enigma in Antarctica. Despite being located beneath several meters of ice, it hosts an entire ecosystem that has never been seen before.