Also known as a burning bar, the thermic lance relies on high-temperature combustion of iron to burn its way through metal and even concrete. The lance is typically a 10 to 30-ft iron or mild-steel tube filled with iron-alloy wires. Magnesium or aluminum in the mix can bump the flame temperature from 5,400 to 8,000°F.

One end of the lance is attached to an oxygen tank. The flow of gas creates a combustion-friendly environment, with the metal itself as one of the reactants. The robbers used a welding torch to get the metal glowing, although many of today’s systems use a self-contained electrical arc.

Once the reaction is underway, the high-temperature flame can shoot sparks and molten metal up to 6 ft from the end of the lance, depending on the oxygen’s flow rate.

This flame easily cuts through steel, which melts at around 2,700°F. The lance’s temperature is even higher than the 4,500°F needed to melt concrete, making the lance an ideal tool for tackling concrete with steel-reinforcing bars. However, the lance is more suited to boring 2-in. holes than slicing a clean line through concrete. Cutting a 2-in. hole in a standard 18-in.-thick vault wall can take up to 3 hr. And a job that size would require several lance reloads and additional oxygen as both are consumed by the flame.

Since the 1970s, newly constructed vault walls have slimmed to as little as 3 in., but concrete formulations can now be 10 times stronger, making cutting even slower. Other innovations such as glass layers and heat-wicking liners to resist melt-through may foil today’s bank robbers. And electronic advances like automatic sprinklers, motion detectors, and closed-circuit cameras that didn’t exist in 1971 present further obstacles to modern-day heists.