Marine turtles were considered as mute under water.

Marine turtles were considered as mute under water, but Isabelle Charrier specialist in bioacoustics at the NeuroPSi Institute, showed in a recent study that juvenile green turtles are able to produce 10 types of sounds under water. Production mechanism and the biological functions of these sounds are still unknown.
See the press release in Ouest France journal (in french)

Credit: Fabien Lefebvre