Humpback whale songs are absolutely haunting, but here's a marine mammal whose call is even more alien. This is the Weddell seal (Leptonychotes weddellii), an Antarctic pinniped whose vocalizations both underwater and top side resemble ambient techno. These pulses and chirps have territorial and mating functions and will probably give you chills.
You can listen to some underwater Weddell seal calls above, and hear some adults (and a pup!) vocalizing on ice in the video at left.