National September 11 Memorial Museum opens in New York
15 May 2014 Last updated at 02:20 BST
The National September 11 Memorial Museum tells the stories of the more than 2,700 people who died in the city when jet aeroplanes hijacked by Islamist terrorist destroyed the World Trade Center.
It also tells of those who survived, and of how the world has changed since the attack.
Below the September 11 memorial plaza, with its two large pools at the footprints of the fallen towers, the museum reaches down to bedrock where the towers' steel columns were anchored.
The BBC took a tour before the official opening and spoke with a relative of one of the victims as well as the museum's director.
Produced by the BBC's Anna Bressanin