How long did it take the Twin Towers to collapse?

The South Tower collapsed 56 minutes after being hit, and the

North Tower 102 minutes after being hit. Once the buildings began

to fall, each took less than 30 seconds to fully collapse. The

South Tower, though hit second, collapsed first.

Parts of the South Tower reached the ground about 9 seconds

after its collapse began at 9:59 AM EDT, approximately 56 minutes

after being hit. The central core of the lower 40 floors caved in

within 25 seconds thereafter. Debris flattened or filled the seven

basement levels within a few seconds.

The TV mast and roof from the North Tower struck the ground

about 11 seconds after the descent began, at 10:28 AM EDT, 102

minutes after being hit. The central core of the lower 60 floors

completely collapsed within 10 to 15 seconds after being bypassed

by the falling upper floors.

Most of the interior collapse was hidden by dust and debris,

which also spread over the surrounding buildings and streets.