I’ve been playing around a bit with an Aquarian H2a hydrophone that I recently added to my collection of mics.
Image from Aqaurian audio
Below you can hear two recordings of heartbeats made with the contact mic adapter attached.
In the last recording presented here the hydrophone with the attached contact mic adapter were placed at the bottom of a sink full of water that was drained.
Geek note: Aquarian H2a (contact mic adapter attached, xlr -> 3,5 mm adapter used) -> Sony PCM-D50 (plug in power on)