The archaeologists who excavated the tombs of the pharaohs were lucky. When they stumbled upon the remains of an ancient civilization, they found gold and valuable artefacts. Their descendents will not be so fortunate. When explorers go digging for our last remains, what they find may be valuable, and it will certainly tell them something interesting about our culture. But it could also kill them, because the longest lasting monuments of our civilization will probably be our nuclear-waste repositories, and the radioactive “treasure” they harbour will remain dangerous for thousands of years.

What does this say about us? This is the central question posed by the film Into Eternity – a fascinating and troubling documentary about a waste repository in southwest Finland called Onkalo, a name that means “hiding place”. Currently under construction, Onkalo is due to receive its first consignment of radioactive waste in 2020. When it is completely full, sometime in the early 22nd century, its entrance will be sealed. Its designers hope that it will remain that way for at least 100 000 years. But no human-built structure has ever lasted a 10th of this time, so every decision made about Onkalo rests on uncertain ground.

Subtitled “A film for the future”, Into Eternity explores this uncertainty in detail. The film, which will get its UK première in Sheffield at the Doc/Fest event in November, discusses the physics of radioactivity, the practicalities of interim and permanent storage, the requirements of the law, and the vexed question of how to keep our descendents safe from Onkalo. Between interviews with various Finnish and Swedish officials, filmmaker Michael Madsen takes us round the Onkalo site, including the unfinished tunnel, which will eventually stretch for 5 km and reach depths of more than 450 m.

The tunnel is a surreal place, covered in unintelligible markings and suffused with a dim blue light. One interviewee – a workman called Sami Savonrinne – likens it to a time capsule. We hear Savonrinne’s words as he crouches on the tunnel floor, a lonely figure in a high-visibility jacket preparing to blast away the next section of bedrock (see image). It is a striking image, one of many in this surprisingly beautiful film. The music is also well chosen, with a multinational soundtrack featuring music by the Finnish composer Jean Sibelius as well as Arvo Pärt, Kraftwerk and – to great effect, in the film’s final scene – Edgard Varèse.

Such artistry would be wasted if the interviews did not provide content to match. Fortunately, Madsen has put together a remarkably candid bunch of experts – some affiliated with Onkalo, others not – and they all have interesting things to say. One of the most fascinating discussions concerns the chances of Onkalo being found, and the consequences of any such “human intrusion”. The experts generally agree that the repository will, at some point, be forgotten – certainly by the next predicted ice age in 60,000 years, and probably well before then. As a result, says Onkalo’s senior manager of communications Timo Seppälä, “My personal belief is that no human intrusion will take place at any timescale ever.”

Timo Älkäs, the facility’s vice president for engineering, is more equivocal. Someone might break into Onkalo, he concedes, but if they did, they would have tools to measure the radiation. One of the external experts, Peter Wikberg of Sweden’s Nuclear Fuel and Waste Management Company, elaborates on Älkäs’ point: any civilization advanced enough to dig into Onkalo, he says, would also be advanced enough to know what it was dealing with.

That is a comforting thought, but his colleague Berit Lundqvist immediately casts doubt on it, noting that 16th-century Swedish miners were able to dig several hundred metres below the surface even though they were unfamiliar with steam engines, let alone radioactivity. Over such an immense stretch of time, we cannot assume that humankind will become ever more technologically advanced; any number of events could send our descendents back to the Middle Ages. The moderate-technology society that might follow is a nightmare scenario for Onkalo’s designers, one where “people may drill but may not understand”, concludes Mikael Jensen, an analyst with Sweden’s Radiation Safety Authority.

Would it help to warn them? Possibly – but there is no guarantee that a warning would be understood. Even if it is, the advice might not be heeded. As the film points out, one Norwegian rune stone, carved less than 1000 years ago, bears a warning that it “should not be touched by misguided men”. The stone was found lying face down.

Yet Finnish law states that the future must be informed, so it will be – in Finnish-language archives that are unlikely to last more than a fraction of Onkalo’s useful life. In the film, the task of explaining this legal lunacy falls largely to Esko Roukola, principal advisor for regulation at Finland’s Radiation and Nuclear Safety Authority. He looks distinctly uncomfortable about it. Asked if he trusts future generations, at first he squirms and waves the camera away. Eventually, he stammers “I cannot say that I trust but I cannot say that I don’t trust.” It is one of the film’s best lines, succinctly capturing the problem Onkalo’s builders face.

There are a few gaps in the film, mostly on the technical side. For a place that is meant to be stable and unchanging, the Onkalo tunnel appears to contain an awful lot of running water. It would have been nice to hear at least one expert explain in more detail how waste is to be kept segregated from groundwater over the next several thousand years. A more nuanced approach to the facility’s 100,000-year lifespan would also have been welcome. Half-lives being what they are, at some point Onkalo’s waste, though still hazardous, will no longer pose an immediate threat to life. How long will that take? 500 years? 1000? 10,000? The film does not say.

On a related note, it is a pity that Madsen’s interviewees give short shrift to the possibility of transmuting waste into less hazardous substances with shorter half-lives. Although Juhani Vira, Onkalo’s senior vice president for research, accurately points out that transmutation would not make all the waste disappear, it would certainly reduce the total volume and perhaps the required isolation period. This is not a small advantage. Building a handful of Onkalos to last 1000 years would be a manageable engineering problem. Building several hundred to last 100,000 seems dangerously close to a crime against the future.

Unfortunately, there does not seem to be an alternative: if we want nuclear power, we will get nuclear waste. Indeed, we have accumulated more than 200,000 tonnes of waste already, so even if we shut down all our nuclear power plants tomorrow, we would still have a massive problem. Places like Onkalo represent an implicit promise that we can keep this waste safe – not only in our own time, but for what might as well be an eternity. So are they the solution? Into Eternity has no answers, but it is a beautiful film about an ugly problem, and anyone interested in nuclear power should see it.