The Tunnel to Towers climb sold out for the second year in a row. The first wave began at 5 a.m.
Participants are scaling more than 2,200 steps to the top of One World Trade, simulating the ascent of brave first responders on 9-11.
Finishers get to enjoy a sunrise panoramic view of Lower Manhattan.
The Tunnel to Towers Foundation was founded in honor of firefighter Stephen Siller, who ran from the Brooklyn Battery Tunnel to the World Trade Center on 9-11, where he died saving others.
Money raised from the event will go toward helping injured veterans and first responders.
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