The biggest advantage of submarines is that they can dive into the water. Once the submarine dives into the water, it is difficult for the naked eye and radar equipment to detect it, so sonar equipment can only be used. Especially modern submarines, can dive more than 500 meters. It is very difficult to detect submarines in the ocean with sonar facilities on the water. There are various restrictions on installing sonar equipment underwater. Just say that other countries won't let you go. So submarines are hard to find.