Deep beneath the forests of Finland, engineers are preparing to activate a facility that sits inside one of the most geologically stable places on Earth—a bedrock formation near Olkiluoto estimated to be around 1.9 billion years old. The project, known as Onkalo, is the world’s first full-scale permanent deep geological repository for high-level nuclear waste. It is built roughly 400–450 metres underground, where spent nuclear fuel will be sealed in copper canisters and surrounded by bentonite clay, a material designed to block groundwater movement and contain radiation for extremely long time periods.
The idea behind choosing such ancient rock is simple but profound: stability over geological time. This bedrock has already survived ice ages, tectonic shifts, and major climate changes with very little movement, making it one of the most secure natural environments known. Once operational, the facility is expected to store thousands of tonnes of nuclear waste and then be sealed permanently, with no intention of reopening it. Scientists describe it as a rare attempt to solve a problem on a 100,000-year timescale, far beyond any modern infrastructure lifespan. But it also raises difficult questions—how to guarantee safety across deep time, and how future generations would even recognise what lies beneath the ground. More