Today marks the 16th anniversary of the worst terrorist attack ever on American soil. Here are five facts you should know about what happened in the aftermath of the events on September 11, 2001:

1. It took 99 days—until December 19, 2001—for thefires at Ground Zeroto be extinguished.Cleanup at Ground Zero wasn’t pleted until May 30, 2002. It took 3.1 million hours of labor to clean up 1.8 million tons of debris at a total cost of cleanup of $750 million.

2.There were 20 people pulled from the rubble in the two days after the attack. On the day following the attacks, 11 people were rescued from the wreckage, including six firefighters and three police officers. Two Port Authority police officers were also rescued after spending nearly 24 hours beneath 30 feet of rubble.

3.In 2010, Congress created theWTC Health Programtoprovidemedical monitoring and treatment for emergency responders, recovery, and cleanup workers, and volunteers who helped after the terrorist attacks.As of2015, the number ofGround Zero responders and others afflicted with 9/11-linked cancers has hit 3,700. Those suffering cancers certified by the WTC Health Programinclude 1,100 members of the New York fire department,2,134 police and first responders, and 467 survivors such as downtown workers and residents. Many have more than one type of cancer.

4.Most of the steel from the World Trade Center wreckage was sent to New Jersey salvage yards where it was broken down and sent all over the world for reuse. Nearly 350,000 tons of the steel was sent to be reused in small and large scale tributes, including 7.5 tons for use in the navy battleshipUSS New York.

5. On September 13 a worker at the site, Frank Silecchia, discovered a 20-foot cross of two steel beams amongst the debris. The beams were dubbed the “Ground Zero Cross” and became a spiritual symbol for families of the victims and workers who cleaned up the debris.