NOT LONG ago a part of the British government asked RAND Europe, a think-tank in Cambridge, England, to conduct some research on undersea critical infrastructure. The think-tank studied publicly available maps of internet and electricity cables. It interviewed experts. It held focus groups. Halfway through the process Ruth Harris, the leader of the project, realised that she had inadvertently unearthed many sensitive details that could be exploited by Russia or other adversaries. When she approached the unnamed government department, they were shocked. The reaction, she recalls, was: “Oh my god. This is secret.” When they learned that Ms Harris’s team was drawn from all over Europe, they demanded that it be overhauled, she says: “This needs to be UK eyes only.”

Western governments have been quietly concerned about the security of undersea cables, which carry most of the world’s internet traffic, for many years. But only recently has the issue come into sharp focus, owing to a series of murky incidents from the Baltic Sea to the Red Sea and a wider realisation that infrastructure, of all sorts, is a target for subversion and sabotage.

Across Europe, Russian spies and their proxies have attacked Ukraine-linked targets, hacking into water utilities, setting fire to warehouses and plotting to strike American military bases in Germany. The fear is that underwater communications could be crippled in a crisis or in wartime, or tapped for secrets in peacetime. And as America and China joust for influence throughout Asia, undersea cables have become a crucial part of their competition.