Inside a nuclear tomb: The underground store that's humanity's first attempt to dispose of nuclear waste for 100,000 years

Onkalo is a sprawling site, with 31 miles of tunnels that reach 1,300 feet under the ground.

An aerial view of the site is annotated to show the location of the Olkiluoto nuclear power plant and of Onkalo.

Here is the way in to the access tunnels, seen from above.

We drove down in a van, deeper and deeper.

The tunnel was just wide enough for one vehicle. I was aware I'd never been this deep underground before.

Signs at regular intervals reminded us how far we'd come — this green one marks 3.1 km, or 1.9 miles, to the surface.

We got a bunch of gear to keep us safe underground.

This is the bottom. These heavy metal doors mark the beginning of the area where the nuclear waste will be held.

Around me were thousands and thousands of feet of rock like this — migmatite with veins of granite.

The rocky chamber will be the final home for nuclear fuel roads like these — metal poles embedded with chunks of refined uranium.

The rods spent their working life stacked by the thousand in a reactor core, making heat that will be turned into electricity. This is me in a (replica!) reactor, with a wall of rods behind.

The rods come out spent, hot, and highly radioactive. Their first stop is 40 years in nearby cooling ponds.

Posiva has a plan after the pool phase. The spent rods — which haven't been in a reactor for 40 years — will go in huge metal tubes like this.

To avoid water getting to the canisters, the next defense is a special type of clay called bentonite.

After being fully sealed, the waste canister will enter an elevator here to be taken down to permanent storage.

This is the other end of the elevator shaft.

An automated digging machine goes along each tunnel to make a shaft for each canister, like in this animation.

This is a prototype of a real-life hole-drilling machine.

Another machine lines the hole with bentonite to protect the canister from water.

This is one of the deposition tunnels in real life. The canisters will be buried into the floor.

A third machine — also automated — picks up a canister from a short-term storage room like this.

And takes it to the prepared hole, lined with bentonite, where it deposits it.

Here is a real-life prototype of the canister-moving machine.

The shaft is then sealed.

As each hole is filled, another machine comes and fills the entire thing with more bentonite.

Each finished tunnel will be sealed further with a massive concrete plug, like this.

Humans will still need to work on the site until it is closed, sometime around 2120.

The cave complex comes with bathrooms, showers, and a cafeteria where workers can take a break.

The workers don't have parties down there... yet!

Engineers are confident they've thought about every feasible possibility.

Mustonen, the Onkalo geologist, says people probably don't realize how much work has gone into future-planning the site against every eventuality. 

"Maybe they ask: have they thought of this?" he said

The answer, he said confidently, is: "Yes, we have." 

Asked if he was concerned that a volcano or earthquake could bring the waste back to the surface, Mustonen simply said: "We have seen that happening in movies." 

But he says "something like that just doesn't happen," he said. 

In terms of geological events, Mustonen says the next big predicted event is an ice age — about 50,000 years from now. This could put pressure on the system by flooding the ground with more water and causing earthquakes. 

But even then, the geologists' work suggests that the site will not be substantially affected. And even in the unlikely event the particles are released into the environment, Posiva's calculations suggest that by then, the risk to humans and the environment will remain very small.

How is Onkalo future-proofing against accidental human exposure? Not with radioactive cats or nuclear warding religious cults, Posiva says.

The world will be looking at Onkalo for inspiration.