The machine that Dr O'Brien and Dr Frazier have designed to get round these constraints consists of a transmitter that sends out a pulse of sound at a frequency of 6 kilohertz, and an array of 52 receivers that collect the reflections from anything buried beneath the surface.

ECONOMIST: Archaeology

That fleeting pulse, a 20-nanosecond flash, can't arrive at once or the tiny round target will warp, making ignition impossible.

FORBES: A Nuclear Hybrid