Rock cores collected from deep beneath the planet's surface suggest ancient microbes have been living inside Europe's largest meteorite crater for millions of years.

Some 400 million years ago, a massive space rock slammed into northern Europe, excavating a giant crater in the middle of what's now Sweden. Today, prospectors are drilling for natural gas within the confines of the ancient crater, the contours of which are called the Siljan Ring.

The drilling attempts have yielded fresh rock cores, several of which made their way to geochemistry labs at Linnaeus University in Sweden.

When scientists at Linnaeus investigated the rocks, they found evidence of long-term deep microbial activity.

"We examined the intensively fractured rock at significant depth in the crater and noted tiny crystals of calcium carbonate and sulphide in the fractures," Linnaeus researcher Henrik Drake said in a news release. "When we analyzed the chemical composition within these crystals it became clear to us that they formed following microbial activity."

"Specifically, the relative abundance of different isotopes of carbon and sulfur within these minerals tells us that microorganisms that produce and consume the greenhouse gas methane have been present, and also microbes that reduce sulfate into sulfide," Drake said. "These are isotopic fingerprints for ancient life."

Using radioisotopic dating techniques, scientists were able to determine that microbes were cycling methane during the formation tiny calcite crystals, and that the activity occurred between 80 and 22 million years ago.

"This marks long-term ancient microbial activity in the impact crater, but also that the microbes lived up to 300 million years after the impact," said Nick Roberts of the British Geological Survey.

Scientists have long theorized that ancient asteroid and comet impacts could have delivered the ingredients for early life forms to Earth, or perhaps even shepherded already thriving microbial communities from distant planets to ours. But the latest research -- published this week in the journal Nature Communications -- suggests the microbes living deep inside the Siljan crater arrived after the impact event.

"At Siljan we see that the crater is colonized but that it has mainly occurred when conditions, such as temperature, became more favorable than at the impact event," Drake said. "The impact structure itself, with a ring zone of down-faulted Paleozoic sediments, has been optimal for deep colonization, because organics and hydrocarbons from shales have migrated throughout the fractured crater and have acted as energy sources for the deep microbial communities."

But while the latest investigation didn't reveal evidence of alien microbes, it did offer proof that impact craters can serve as prime real estate for ancient microbe colonies.

"Our findings indeed confirm that impact craters are favorable microbial habitats on Earth and perhaps beyond," Drake said.

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