One place radar isn't used is to help submarines as they navigate underwater. Electromagnetic waves don't travel readily through dense seawater (that's why it's dark in the deep ocean). Instead, submarines use a very similar system called SONAR (Sound Navigation And Ranging), which uses sound to "see" objects instead of radio waves. Submarines do, however, have radar systems they can use while they're moving about on the ocean surface (such as when they're entering and leaving port).
Ground-penetrating radar being used by scientists
Countermeasures: how to avoid radar
Radar is extremely effective at spotting enemy aircraft and ships—so much so that military scientists had to develop some way around it! If you have a superb radar system, chances are your enemy has one too. If you can spot his airplanes, he can spot yours. So you really need airplanes that can somehow "hide" themselves inside the enemy's radar without being spotted. Stealth technology is designed to do just that. You may have seen the US air force's sinister-looking B2 stealth bomber. Its sharp, angular lines and metal-coated windows are designed to scatter or absorb beams of radio waves so enemy radar operators cannot detect them. A stealth airplane is so effective at doing this that it shows up on a radar screen with no more energy than a small bird!