S1, E2: The Germans, ignoring international treaties they signed, were first to weaponize a chemical. In this case it was chlorine--a highly toxic ingredient used in the manufacture of chemical dyes of which they had a huge supply. Their action unleashed an escalation of poison gas weapons as both sides developed different chemicals and more effective counter measures. Their use of culminated in a bold British plan that unfolded beneath the battlefield at Messine Ridge. British mining and explosive experts planted 450 tons of high explosive hidden in a network of tunnels under Messine ridge. When ignited, they created the biggest land mine in history--killing 10,000 German soldiers. The explosion rattled windows at 10 Downing St., 140 miles away and registered as an earthquake in Switzerland.