How to hide something dangerous for tens of thousands of years

Here’s a puzzle for you: imagine you have enough ‘stuff’ to fill St Paul’s Cathedral to the brim five times over. This stuff is toxic to life on Earth. And it’s going to stay toxic for centuries.

Where are you going to put it?

It’s a question that has vexed experts ever since the world was yanked into the atomic age by US physicist J Robert Oppenheimer around 80 years ago. At first, it was military nuclear tests...