Sometimes it is an unexploded bomb, sometimes a lost anchor or just sometimes part of the missing treasure wreck. Sonar Sonar works by sending out a pulse of sound that then gets reflected back from either the seabed or anything else down there. It was first developed to help warships look for submarines, but nowadays it can also help us look for shipwrecks. The basic kind is an echo sounder, and a paper trace records the seabed. If the bottom is flat, a shipwreck might show up as a bump on the flat seabed.
Sidescan sonars do the same job, but cover a much wider area, sweeping a large lane over the seabed.
Searchers still have to sail their boats up and down, like mowing a lawn, making sure all of their search area is covered. A development of the echo sounder is the sub-bottom profiler. This is an echo sounder powerful enough to look through the ocean floor and give some idea if anything is buried beneath the sand or mud. Other tools have been used, from aircraft, towed sledges, unmanned mini-submarines or just simply using scuba divers. The best tool is knowledge.