The 32.5% ammonium nitrate, used as fertilizer and in high explosives, was manufactured in Nebraska and Iowa and shipped to Texas City by rail before being loaded on the Grandcamp.
It was manufactured in a patented explosives process, mixed with clay, petrolatum, rosin and paraffin to avoid moisture caking. It was also packaged in paper sacks, then transported and stored at temperatures that increased its chemical activity. Longshoremen reported the bags were warm to the touch prior to loading.
Around 08:10, smoke was spotted in the cargo hold of the Grandcamp. Attempts at control failed as a red glow returned after each effort.
Shortly before 9:00 AM, the Captain ordered his men to steam the hold, a firefighting method where steam is piped in to put out fires in the hope of preserving the cargo. Meanwhile, the fire had attracted a crowd of spectators along the shoreline, who believed they were a safe distance away. Spectators noted that the water around the ship was already boiling from the heat, an indication of runaway chemical reactions. The cargo hold and deck began to bulge as the forces increased inside.
At 09:12, the ammonium nitrate reached an explosive threshold and the vessel then detonated, causing great destruction and damage throughout the port. The tremendous blast sent a 15-foot (4.5 m) wave that was detectable over nearly 100 miles (160 km) of the Texas shoreline. The blast leveled nearly 1,000 buildings on land. The Grandcamp explosion destroyed the Monsanto Chemical Company plant and resulted in ignition of refineries and chemical tanks on the waterfront. Falling bales of burning twine added to the damage while the Grandcamp's anchor was hurled across the city. Sightseeing airplanes flying nearby had their wings shorn off , forcing them out of the sky. Ten miles away, people in Galveston were forced to their knees; windows were shattered in Houston, Texas, 40 miles (60 km) away. People felt the shock 250 miles (400 km) away in Louis...