I think these guys may have just a little too much time on their hands.  It’s either that or they’re insane.  They created a double-horn subwoofer under the floor of an audio room.  The excavation for the enclosure was 60 cubic meters in volume.  Each horn is 9.5 meters long and the subwoofer has the ability to output full power at 10 Hz (most modern stereo equipment doesn’t go below 20 Hz because CDs can go lower than this).