One evening last September the P524 Nymfen, a Danish naval vessel that tracks suspicious Russian shipping, made an unexpected detour from its usual patrol route.

For the first time in years, it skirted the Baltic island of Bornholm and made its way northeast to the outer fringe of Denmark’s estimated radar range. It paused for half an hour, turned around and switched off its transponder, vanishing for several hours from international tracking systems.

Minutes later a Swedish air force surveillance aircraft began prowling back and forth nearby and a Swedish corvette set off for the site at considerable speed before circling the surrounding waters.

Four days after that the Nord Stream 1 and 2 pipelines, a highly controversial project built to pump Russian natural gas