(CNN) -- It's one of the great questions of our age: What to do with nuclear waste?

It's challenging, not just because radioactive material is highly toxic, but because really engaging with the problem forces us to confront unimaginable timescales.

But in Finland they believe they have found a solution, with the world's first permanent nuclear-waste repository -- "Onkalo" -- a huge system of underground tunnels that is being hewn out of solid rock and must last at least 100,000 years.

"It is our law that we have to dispose of our nuclear waste within Finland's borders," Timo Seppala, from Posiva, the company constructing the site, told CNN.

"It was also important that we found a solution that would require no surveillance or management by future generations."

The site is situated at Olkiluoto, approximately 185 miles northwest of Helsinki.

Work on the concept behind the facility commenced in 1970s and the repository is expected to be backfilled and decommissioned in the 2100s. None of the 40 people working on the facility today will live to see it completed.

The bunker is based around a spiraling track that will eventually be three miles long, and reach a depth of 500 meters.

"We are at final disposal depth now, about 420 meters," said Seppala.

The waste will be secured through a system of "multiple barriers," the first being the Finnish bedrock itself, the rest engineered from steel and concrete, with the waste fuel rods stored in corrosion-resistant copper canisters with five centimeter-thick walls.

These canisters will then be deposited in a bed of bentonite clay that, because it swells when it absorbs water, will both create a buffer against any geological movement and prevent liquid pooling, which could corrode the copper.

"There are no countries who are as far ahead as we are in this area," said Seppala. "We are going to be the ones who set the standard for final disposal."

The final cost is expected to be €3 billion ($4.1 billion). But since the 1970s the Finnish government has been collecting a clean-up fund from the generating companies via a levy on the price of nuclear power, which is already over €1.7 billion and will be used to fund the final disposal.

Onkalo is the subject of "Into Eternity," a new documentary by Danish filmmaker Michael Madsen, which explores some of the philosophical questions raised by the facility.

"What interested me was how those involved [with Onkalo] responded to the idea of 100,000 years," Madsen told CNN. "That time span is new to humanity.

"What does it say about our civilization when we build something that will last to a time when all we know is gone and forgotten?"

"Into Eternity" is very different to most "eco documentaries," taking as it does a detached view of proceedings, offering viewers fictional myths about how the site may be remembered, alongside interviews with engineers and scientists.

"I had to get beyond talking about it as a technological problem," said Madsen.

"In many ways I think that these engineers are tasked with a much greater problem than they can actually solve.

"You can perhaps build this facility in titanium-reinforced concrete, but that's not the real issue.

"How are we going to stop people getting in there at a point at which all knowledge will have been lost?"

What debate there is centers around whether it is better that Onkalo is remembered -- and all is done to ensure the memory of the danger buried there is passed on, right down to constructing hieroglyphic monoliths to speak pictorially to a time when all current languages are long dead.

Or whether it is better sealed and forgotten.

The fear is that for whatever reason, future generations will start digging, unaware of what is inside.

However, Seppala believes Madsen makes too much of the risk.

He said he enjoyed "Into Eternity" as a piece of art, but he felt it overplayed the consequences of anyone entering Onkalo in the future.

"The film gives the impression that opening Onkalo would be like opening a Pandora's box," he said. "But in reality a few people would be exposed to radiation; it would not be a global catastrophe."

However, Madsen believes there is still uncertainty.

"In the United States they say that the same high-level waste being buried at Onkalo has to be kept safe for a million years," he said.

"It's an open question: How long should we keep this stuff safe? When we start talking in these timescales we just don't know."