Here's the whole story. Aside from the first guy, who is describing stealth or "low observability:, everyone here had described a different aspect of using low altitude to avoid radar. A transmitter's elevation, it's angle compared to the ground, terrain and clutter, and the curvature of the earth all play a role in avoiding radar. "Flying under" usually refers to staying below the beam of the radar. As above answers indicate, how low you have to fly is determined by the angle of the radar and how high it is placed. Putting a radar on a tower, however, allows the operator ro "look" down over terrain and the Earth's curvature, so choosing the best flight path can be tricky. Flying behind mountains and low enough for trees, boulders, dust, etc. block the radar signal is a related, but separate technique called "terrain masking". It's important to note that the mst effective radar arrays are attached to aircraft called AWACS or Airborne Warnng And Control Systems. These planes have the ultimate high ground, allowing them, to see even low flyers. Those and newer, more sophisticated systems like fast scanning phased array radars are making it harder and harder to hide down low.