During World War II, they were primarily developed by nations such as Germany to safeguard vital industry from aircraft assault. Industrial bunkers are also used as control rooms for hazardous tasks such as rocket engine testing or explosive experiments. In modern times, they are used again for protection against aerial attack, this time on military targets. They are also used as safe storage facilities for toxic chemicals. Civilian use includes mining bunker houses where people can work in safety while being protected from underground explosions. During WWII, international law allowed the use of subterranean structures for defense purposes. The Germans used mines as defensive weapons long before it was officially approved under the Geneva Convention. As far back as 1915, German engineers built mine-protected factories to protect their industrial machinery from enemy attacks. These first "industrial bunkers" were simple chambers with no internal support beams or doors that could be filled with high-grade metal ore to shield it from aircraft bombs. Soon after the war started, the British and Americans began building their own bunkers. They were needed because traditional fortifications were not giving good results against air raids. The Germans used hydrogen gas during the battle of Britain because it was invisible to radar and could travel farther than conventional explosives. This would not be possible without first encasing them in protective shells.