In the beautiful Brittany region of France, there’s a small town called Plougastel-Daoulas. You’ve almost certainly never heard of it, and we wouldn’t blame you. It’s a tiny, not particularly celebrated part of the country – hardly Paris or Nice. But in 2019 a remarkable mystery finally put Plougastel-Daoulas on the map. Suddenly every puzzle expert worth their salt knew where it was – and, crucially, of the strange riddle that lies in a cove there.