After a Soviet submarine ran aground on the Swedish coast in 1981, the Swedish government encouraged people to report suspicious sightings. Thousands of reports came in. But Sweden had a problem. The military did not have enough resources to patrol the country’s long coastline. So instead, it used underwater microphones to listen for Soviet submarines. The name of the two researchers mentioned in the video are Magnus Wahlberg and Håkan Westerberg. Sources: Polejack, Andrei, Paulina Ramírez-Monsa

Hop On Hop Off Bus Tour in Gothenburg, Sweden