Now they’ve got sonar buoys out there, there may be some other hydrophones that the mothership the Polar Prince had in the water.

Source: https://www.express.co.uk/news/us/1782830/Titanic-sub-employee-sacked-raising-safety-concerns

Over the last three years, some of this cable network has been expanded and replaced with advanced cables fitted with state-of-the-art hydrophones and sensors to more accurately pinpoint the location of enemy vessels, the two people said.

Source: https://gcaptain.com/u-s-revives-cold-war-submarine-spy-program-to-counter-china/