Inventions etc

Zel, I would love a whole book or video on using a depth finder.  The best articles that I have read about them are on this site but a book or video would be fantastic!  

The fish that you see on Bassmasters have a sonar (sound) transmitter on them.  They just have to put a different frequency sound on the four different fish to distinguish between each of them.  To find the fish they have to put at least two microphones in the water and triangulate on the fish.  

GPS uses RF (radio frequencies) which cannot penatrate water.  Submarines have to rise to the surface or raise an antenna to talk to the outside world because of this problem.