The European Space Agency’s Star Tiger project has taken terahertz images of a human hand. Some of the pictures show just how useful the imagery might be for peering through walls and such – one of the images is through a 15mm pad of paper.

One terahertz is 1,000 gigahertz, and most current radio technology stops at around 100 GHz: 0.1THz. Everything gives off terahertz radiation naturally, and like radio waves — but unlike heat or light — the waves can pass through some solid objects. Like light, it is possible to focus the radiation and create images as if the intervening material were translucent, and by analysing the frequencies given off the chemical and physical characteristics of the object can be worked out.
More here.