How did the bottom of Lake Huron get riddled with sinkholes that time forgot? Find out, and see a video of life that hasn’t existed for billions of years.

The sinkholes at the bottom of the lake are pockets of de-oxygenated water that have pooled beneath the fresh waters above. So all the creatures who live in the sinkholes might have evolved at a time on Earth when no oxygen was available. In a recent Earth magazine article about the ongoing exploration of these sinkholes, first discovered a little over a decade ago, Lindsey Doermann writes:

These pockets of water teem with microbial life similar to that found around deep ocean hydrothermal vents or beneath ice-covered Antarctic lakes, not the kinds of microorganisms normally found in our own backyards . . . Before long, the true importance of these oddities became apparent: “These ecosystems in Lake Huron are analogs of the Proterozoic,” says Bopi Biddanda, a microbial ecologist at Grand Valley State University in Michigan and one of the leaders of the sinkhole science team. “They could be windows into communities that existed 3 billion years ago.”

Though it’s possible the creatures in the sinkholes are direct descendants of creatures who lived on pre-oxygen Earth, it’s more likely that they evolved from more recent organisms to thrive in an oxygen-free ecological niche. Either way, they are the closest we’ll get to seeing life from Earth’s distant past.

But how did those sinkholes get there in the first place? As you can see from this diagram, what’s happened is that freshwater from the Earth’s surface has sunk below ground, and eventually worked its way back out beneath the lake. As the water slowly eroded the lakebed, it created sinkholes of freshwater — lakes within lakes, if you will — where anaerobic or oxygen-free ecosystems began to thrive.

In this incredible video of the sinkholes, you can see the oxygen-free freshwater bubbling up from the lake bottom. All life on Earth may have once resembled these strange, algae-furred fingers reaching up from the sinkholes. It wasn’t until about 2 billion years ago that the planet began to have a significant amount of oxygen in its atmosphere. The shift to oxygen was caused by cyanobacteria like what you see in this video, who emit oxygen as part of their digestive cycle. As cyanobacteria took over ancient Earth’s seas, geologists believe that they caused the planet’s first climate disaster, killing off all the life forms that didn’t metabolize oxygen. Essentially, oxygen was a poison gas to them and made the planet unlivable. Lucky for multicellular organisms, oxygen ushered in a new era where life proliferated and diversified in dramatic ways, eventually leading to the world we live in today.