A few years ago, construction for the 2012 Summer Olympics in London unearthed a gift from the Nazis: a 2,000-pound unexploded bomb. Such finds are common—during World War II, London was pummeled with 19,000 tons of bombs—and vintage ordnance can still blow (in 2010, three bomb-disposal experts were killed while digging up a 65-year-old 1,000-pounder in Germany). So UK projects often begin with a call to an outfit like Zetica, a leader in sniffing out subterranean munitions. Each year, Zetica finds more than 8,000 shells, bombs, and mortars in the UK alone. Here’s how.

• 1 Research Targets

  One-tenth of the bombs dropped on the UK never detonated. Luckily, they left a paper trail. Zetica pores over vintage air-raid info and aerial photos in libraries and record offices to ID high-risk zones—mostly old industrial areas and known Luftwaffe flight paths.