'Towfish' used in MH370 hunt lost after volcano crash - CNN.com

The sonar was being towed behind the Fugro Discovery when it collied with a underwater volcano, resulting in it's cable breaking, sending the vehicle and 4.5 kilometers (2.8 miles) of cable to the ocean floor. Authorities believe they can recover the sonar. The Fugro is in route to have replacement cable installed and to pick up a new sonar to aid in recovery. Sonars are one of the most effective tools for underwater exploration because it can search a large area quickly and produce a detailed picture of anything on the bottom, regardless of water clarity.