The Sept. 11 museum opens to the public May 21, preceded by a dedication ceremony Thursday. Five things to know about the museum:

1. Its mission

The exhibits tell the stories of the nearly 3,000 people killed in the 2001 attacks and the 1993 trade centre bombing, as well as of survivors and first responders. Museum Director Alice Greenwald said the museum is "about understanding our shared humanity," while former mayor Michael Bloomberg called it a reminder "that freedom is not free."

2. Museum's size

The museum occupies 110,000 square feet on the 16-acre trade centre site, tracing the foundations of the twin towers 21 metres underground.

3. Construction and foundation

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Nearly 2,000 oral histories in the museum give voice to the memories of survivors, first responders, victims' relatives and others.

Below the Sept. 11 memorial plaza, with its two fountains outlining the footprints of the towers, the museum reaches down to bedrock, where the towers' steel columns were anchored. It's bounded by a slurry wall that kept back the Hudson River after the attack.

4. Costs

The plaza and museum together cost $700 million to build, subsidized with $390 million in tax-funded grants; officials hope the $24 museum entrance fee expected to generate about $40 million a year will help cover operating costs, expected to be about $60 million a year. Fundraising will cover the rest, for now.

5. Special artifact

Among the more than 10,000 artifacts, 23,000 still images and 500 hours of video and film, plus 1,970 oral histories, one special item is what Patricia Reilly looked for among the displays during an earlier tour for families: her sister's picture ID card from the 101st floor office in the south tower where she died.