For almost ten years, Danish musicians Laila Skovmand and Robert Karlsson have been researching, creating and experimenting with custom-made underwater instruments (you can see some of their earlier experiments on YouTube) — including an underwater organ (hydraulophone), a crystallophone, a rotachorda (inspired by a hurdy gurdy), and a setiphone (amplified note bars). They've even developed a special technique for underwater singing.
The culminating result is AquaSonic. Together with the ensemble Between Music (Dea Marie Kjeldsen, Nanna Bech and Morten Poulsen) they've created an underwater concert and installation — performers submerge themselves in aquariums to perform hauntingly turbulent, beautiful works.
See the group perform in the video below, in a performance from the SPOT Festival in Aarhus, Denmark in May of 2015.