Inside the Underground Tunnel That Will Be Sealed for 100,000 Years

Imagine a place no human will step foot in for an astonishing 100,000 years—this is set to become a reality and mark a historic world first.

To put this into perspective, 100,000 years ago, woolly mammoths still roamed the Earth, and the harsh conditions of the Ice Age saw the first humans migrating and struggling to survive.

What the future holds for humanity 100,000 years from now is anyone’s guess—perhaps we’ll have left Earth entirely by then.

But if humans are still here, the time will finally come to enter an underground tunnel in Finland that will remain off-limits to humans for around 4,000 generations.

Finland, a northern European nation bordered by Sweden, Norway, and Russia, has created the world’s first geological repository—a monumental project designed to safely bury spent nuclear fuel.

While the concept may sound unnerving, Finland has earned praise globally, with many hailing it as ‘a model for the entire world’ in terms of sustainable nuclear energy storage.

“A tour at Onkalo, which lies 450m (1,480ft) below the ground, to see tunnels hewn in the living rock to store highly radioactive waste for 100,000 years, suddenly makes me nervous,” writes BBC journalist Erika Benke, who was given the rare chance to tour the facility.

In 2025, this site will be sealed off permanently, and no human will be permitted to enter for 100,000 years.

This ambitious project is the brainchild of Posiva, Finland’s nuclear waste management organization, which plans to encase the spent fuel in watertight canisters and deposit them deep within the Onkalo geologic repository.

A deposition hole, approximately 8m deep and 1.75 m in diameter, is pictured in the Repository in ONKALO, a deep geological disposal underground facility designed to safely store nuclear wasteJONATHAN NACKSTRAND/AFP via Getty Images
Situated in southwestern Finland, the repository is located near the Olkiluoto nuclear power plant.

Benke described passing through a security gate with a turnstile before being escorted into a car by her guide.

“It takes 15 minutes to drive down to Onkalo’s service station which lies 437m (1,430ft) below the ground. As the 4.5km-long (2.8 mile) tunnel begins to snake down, we see a standard traffic sign for a 20km/h (12mph) speed limit. There are also green signs on the tunnel wall at regular intervals indicating how far we are from the surface,” she notes.

A car drives in the access tunnel of Posiva’s spent nuclear fuel repository Onkalo back in 2017Antti Yrjonen/NurPhoto via Getty Images
Benke elaborated on how the tunnel was quite narrow, allowing space for only one vehicle at a time. She also detailed how nuclear waste canisters would be transported: they would arrive at a service area via a lift descending from the encapsulation plant on the surface.

She added: “When the spent fuel starts being stored here, canisters will be lowered from this lift landing area further down to a deposition tunnel where they’ll be picked up by robotic vehicles to take them to vertical deposition holes – their final resting places.”

“We’re shown a demonstration deposition tunnel. Its entrance is a lot darker than the service area and the floor is uneven and wet, muddy in places. The walls are bare bedrock that glints in the torchlight.”

From next year onward, the rest of this incredible site will remain an enigma, sealed away for millennia.

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