A couple days ago we deployed the final mooring of this cruise: a large receiving array located inside the pentagon of our set of source moorings. This array contained 150 hydrophone modules that are set up to listen to the sources.
The picture below shows what a hydrophone module looks like when it is opened up:
The silver colored case on the left is the pressure case. It is what keeps the electronics (the parts on the right) from being crushed when the module is deployed deep in the ocean. On the right side, you can see the lithium battery that powers the module during the year it sits in the ocean. The part labeled "inductive modem enables communication between the module and a control unit located above it on the array. The control unit tells the module when it should turn on and listen to the sources. Of course the module has to have someplace to record the sounds it hears from the sources. An SD card (like the one you probably have in your digital camera) is used to store the recordings. The hydrophone (the underwater microphone) is inside the blue tube at the bottom of the module.
Before we could deploy all these modules, we had to run a series of tests on them to make sure they worked properly. The following video clip shows how we tested that the hydrophone was working before we sealed everything up.