An explosion threat at an aerospace facility in California forced roughly 40,000 people to evacuate their homes for several days. Emergency crews struggled to cool a damaged tank holding the toxic and highly flammable chemical methyl methacrylate. The dangerous buildup of pressure was finally relieved by a crack in the tank, allowing firefighters to safely eliminate the explosion risk without any leaks. With the immediate danger passed and residents safe, authorities have now launched an investigation into the incident that prompted the state governor to request federal emergency assistance.