THE ESCALATING grey-zone conflict between the West and Russia is being contested on the sea as well as in the air. On September 19th, as Russian MiG-31 jets violated Estonian airspace, your correspondent was with Estonia’s navy in its base in Tallinn, the NATO member’s capital. Its naval force is on the front lines of a push to confront the world’s shadow fleet of vessels—ships that conceal themselves or their identities. The number of such vessels has soared from 200 in 2022 to about a thousand today. Some are suspected of going far beyond smuggling sanctioned Russian oil, to engaging in espionage and sabotage across northern Europe.