The Sept. 11 museum 9/11 opens to the public May 21, preceded by a ceremony Thursday that’s to include President Barack Obama, families and other officials. Five things to know about the museum:

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ITS MISSION

The exhibits tell the stories of the more than 2,700 people who died in New York, those who survived and how the world has changed since the attack. 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.”

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MUSEUM’S SIZE

The museum occupies 110,000 square feet on the 16-acre trade center site, tracing the foundations of the twin towers 70 feet underground.

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CONSTRUCTION AND FOUNDATION

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.

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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.

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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.

(AP)