They come from Hell, are tens of thousands of years old, and scientists have not seen anything quite like them before.

An extraordinary population of microbes has been discovered trapped in crystal in a volcanically heated Mexican cave system where temperatures reach 60C (140F).

Some of them had been there for 60,000 years, shut off from light or oxygen and obtaining energy from minerals.

Attempts to classify the bugs showed that 90% could not be matched with any other micro-organisms catalogued in available databases.

The population of microbes has been discovered trapped in crystal (Penelope J Boston/PA)

They were also highly diverse, including around 100 different strains made up both of bacteria and other microbes known as archaea.

Five cave chambers, ranging from the size of an average room to a cathedral-like cavern, were explored at the Naica mine in Chihuahua. The microbes were contained in small water-filled pockets within the sparkling white crystals, some of which were five metres long and a metre wide.

Dr Penelope Boston, director of Nasa’s Astrobiology Institute, who led the first expedition in 2008, said: “The deepest part we accessed was a place called Hell, very evocatively. That chamber is at the 800 metre level. It was a transformative experience … it really felt strange. It was a very hard environment to work in, but tear-inducingly beautiful. It’s like being inside a geode.”

Scientists wore special suits to keep them cool (Penelope J Boston/PA)

To protect them from the heat and allow them to breathe, the scientists wore all-covering “space suits”.

Experiments conducted in situ so see if the weird bugs could be cultured were partly successful.

“Much to my surprise we got the things to grow,” said Dr Boston, who was speaking at the American Association for the Advancement of Science (AAAS) annual meeting in Boston, Massachusetts. “It was laborious. We lost some of them – that’s just the game. They’ve got needs we can’t fulfill. That part of it was really like zoo-keeping.”

The microbes are thought to have obtained energy from minerals (Penelope J Boston/PA)

She said the bugs were as different to known micro-organisms as humans and fungi.

Dr Boston added: “They’re really showing us what our kind of life can do in terms of manipulating materials. These guys are living in an environment where there’s not organic food as we understand it.

“They’re an example, at very high temperatures, of organisms making their living essentially by munching down inorganic minerals and compounds. This is maybe the deep history of our life here.”

The bugs may be a foretaste of what to expect if alien microbes are found on Mars or Jupiter’s moon Europa, which has a global ice-covered ocean.