After 24,000 years, frozen “zombie” worms were revived by Russian scientists.

For about 50 million years, microscopic, multi-cellular animals known as bdelloid rotifers have lived in freshwater environments. Researchers have discovered that the newly thawed animals, which reproduced asexually, had no obstacle to cloning themselves once resurrected.

Other research on rotifers expressed that the critters could recover after ten years on ice. Still, a new study published in the journal Current Biology on Monday, June 7, claimed that they were intelligent. They have the ability to survive for many more millions of years.

According to a Live Science report, these “zombie” rotifers weren’t the first once-frozen animals scientists have defrosted and brought back to life. For example, nematodes—another worm-like species—had been reborn after 42,000 years caught in permafrost. Old trees had also been restored and grown in the laboratory

Stas Malavin, a researcher at the Institute for Physicochemical and Biological Problems in Soil Sciences in the Russian city of Pushchino, told Live Science that these animals could “suspend their metabolism and accumulate certain compounds like chaperone proteins that help them to recover from cryptobiosis when the conditions improve.”

The recuperation process was simple.

Malavin said that they put a piece of permafrost in a Petri dish filled with a suitable medium and waited for living creatures to restore from their dormancy, start to move, and multiply.

While the discovery was important for cryobiology—studying how organisms live in the severe cold—Malavin points out that we were still a long way from doing it to people.

He told Live Science readers that the more sophisticated the organism, the more difficult it is to keep it alive frozen. It wasn’t a process currently possible for mammals.

“The more complex the organism, the trickier it is to preserve it alive frozen,” he assured Live Science readers. “For mammals, it’s not currently possible.”