Self-sealing rock

“Stay calm in the event of an alarm.” This one sentence justbefore the start of the tour sounds more dramatic than anything that follows.

First, we have to put on a helmet, a high-visibility safetyvest, and a tracking device to make sure none of us get lost. The helmet is toolarge for me, and the vest is baggy. I feel like I am dressed up as aconstruction site worker in a bad comedy show when I was really hoping toproject scientific precision. But I forget all about that once the enginesstart and the cars slowly drive into the mountain.

Hopingto be awed

This is my first visit to an underground research laboratory– and also my first direct insight into the research surrounding the planneddeep geological repository for radioactive waste in Switzerland. Myexpectations are accordingly high – surely Mont Terri will reveal somethingspectacular. Perhaps some mystery hidden deep inside the mountain, or enigmatictunnels.

But to be honest: inside the mountain, 300 metres below thesurface, things seem surprisingly normal.

Nothingbut a motorway tunnel?

The tunnel resembles any motorway tunnel. Artificial light.Concrete. Vehicles. People at work. In some places, the rock is secured withraw concrete, cables run along the walls and a machine hums in the background.

But perhaps that is precisely what is so fascinating: thereal hero down here draws no attention to itself. It does its job withoutfanfare and without any indication of how important its role is. The OpalinusClay.

Pinningour hopes on ancient rock

The Opalinus Clay is a grey-brown stone, around 175 millionyears old, older than the Alps. It is not particularly pretty. Neither is itparticularly hard. Tunnel engineers view it as challenging because it candeform or crumble under construction.

And yet it has been selected to do what engineered barriersalone cannot do: safely isolate the radioactive waste for hundreds of thousandsof years – as an underground geological barrier. The Opalinus Clay is verytight and can confine radioactive substances for very long time periods. And itcan seal itself should a fissure occur.

When water penetrates the Opalinus Clay, it start to swell,which seals the fissures again. Last but not least: radioactive particleseffectively stick to the Opalinus Clay, which is an additional safetymechanism.

These properties are just a few of the reasons why researchhas been conducted here at the Mont Terri Rock Laboratory over the past 30years – together with universities and research institutes from variouscountries.

Suddenrise in temperature

Underground, you quickly lose a sense of the outdoors. Thereis no daylight. No mobile phone reception. No wind. The air smells heavy andmineral. It’s not a bad smell, but a thick one. The temperature is a constant13 degrees.

Until it suddenly gets noticeably warmer. This is whereresearchers are investigating how the Opalinus Clay reacts to heat. Radioactivewaste emits heat, and we have to make sure that the rock’s properties alsofunction when it gets hot. Does it get drier, or perhaps more brittle?

Fascinatinglyunspectacular

After about an hour, we head back outside where we findourselves surrounded by rain rather than tunnel walls. I feel that I witnessedsomething special that might not seem all that special at first glance: a rockthat can self-seal so that it protects the deep geological repository – andeventually us, too.

About me

My name is Michèle Vaterlaus and I have been working as acommunication specialist at Nagra since May 2026. I find working withchallenging and complex topics particularly exciting, as it requires clear andcareful communication. This is exactly what makes Nagra so interesting for me.

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