A wide variety of animals have been found living more than a kilometre underground — far deeper than scientists used to think it was possible for life to thrive.

A new study, published recently in Nature Communications, has discovered 17 species of animals living in water trapped in rock as deep as 1.4 kilometres underground in two South African gold mines. They include:

A wide variety of worms — flatworms, roundworms, and ringed worms related to earthworms.

Aquatic organisms called rotifers.

A copepod, a tiny relative of shrimp.

Some of the same species were found in both mines.

The 17 species found deep underground include a) a flatworm b) a segmented worm c) a roundworm and d) a copepod. (Borgonie et al./Nature Communications)

The discovery of such animals so deep underground "is promising for the search for life on other planets/moons in our solar system," the researchers wrote.

Microbes had previously been found as deep as three kilometres underground, said Barbara Sherwood Lollar, a geochemist at the University of Toronto who co-authored the paper.

"Here, we're actually finding the next level of the food web … animals that are feeding on those single-celled organisms," she told CBC's Quirks & Quarks in an interview that airs Saturday.

On the other hand, these complex, higher organisms weren't so big themselves. "You need microscopes to see these things."

Until now, only one complex organism had ever been discovered living so deep — a worm found in 2011 and nicknamed the "worm from hell."

That study was led by Gaetan Borgonie, a researcher with Extreme Life Isyensya, a non-profit group that hunts for life deep underground, and the University of the Free State in South Africa.

He thought other invertebrates should have been able to survive in similar conditions, so he and his collaborators spent two years looking for them in mines that had water trapped in their rocks.

"As we all know, if there's water, there's the potential for life," Sherwood Lollar said.

The animals discovered in the new study are all species known from the surface, except for the copepod, which may be a new species.

That led to the question of how long they had been underground.

Sherwood Lollar analyzed the chemical signatures of the water the animals were found and determined that it had been underground for as long as 12,300 years.

She said the researchers are now seeing if they can find animals in even deeper, older water.