A large tank at an aerospace factory in Garden Grove started leaking. The chemical inside - methyl methacrylate, or MMA - is a clear liquid used to make airplane canopies and strong glues for aircraft parts. When air got in through the leak, the chemical began to harden and turn solid…just like superglue or epoxy does when you leave the tube open. This hardening process creates a lot of heat. With thousands of gallons reacting at once, the heat keeps building and speeds up the reaction even more