National Geographic : 2003 Nov
SNIFFING OUT STOWAWAYS New solutions frequently beget new problems, and the Channel Tunnel connecting France and Great Britain was no exception. In1994 the countries were finally linked by a half-hour train ride beneath the Strait of Dover, mak ing it easy for passengers and freight to cross the border. With up to 5,000 trucks carried through the tunnel each day, it was also easier for undocumented aliens to sneak through. Enter passive millimeter wave imaging (PMMW), a technology that can see through solid surfaces by measuring reflections of natural background radiation. After a PMMW scanner checks moving trucks for bodily forms (above), a "sniffer" (below) measures carbon diox ide levels, elevated by human respiration. With this one-two punch, agents routinely catch stowaways (right).