out -- called tow fish. what the tow fish does, hang onto a cable. it can be as long as 55,000 feet. >> 55,000 feet of cable. >> to drag this. if you think about the angle from the ship that's towing it, this has to be way down here to get to 6,000 feet. >> they have like 10 miles of cable behind them dragging this thing to get to the required depth. the longer the tow line, the deeper under water, better chance of hearing what may be very deep in the water. >> correct. but there's real challenges with that. you have that much cable. you think about towing that, all that distance, five miles behind you. try making a turn when you've got something five miles behind you. >> sophistication in that. they are asking for those. they are asking for sonar buoys.