Part of the 9/11 Memorial site being built in lower Manhattan, New York, in the footsteps of the former Twin Towers, which were demolished by al-Qaeda airliner hijackers in September 2001. The centrepiece is twin pools of falling water - a visual and audible witness to the atrocity, and an earlier attack on the World Trade Centre, in which several thousand people died. The pools' surrounds have on them the names of those who died.