Quick answer
The 9/11 Memorial Museum sits beneath the memorial plaza at the World Trade Center site, where the twin towers once stood. Opened in May 2014, it descends past the surviving slurry wall to the Last Column, the final piece of steel removed from Ground Zero, and remembers the 2,983 people killed in the 2001 and 1993 attacks.
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The wall that held back the river
Below the plaza, the museum's Foundation Hall is built against its oldest artifact: the slurry wall, a concrete retaining wall sunk into the ground in the 1960s to keep the Hudson River out of the excavation. On September 11, 2001, the towers came down, and the wall held. You walk down the ramps into the cavity where the towers stood, past the wall that kept the river back, and you understand why the museum is underground: the place itself is the exhibit. The museum opened in May 2014, and the descent takes minutes; the weight of what happened here takes longer.
The last column
At the center of Foundation Hall stands the Last Column — the final piece of World Trade Center steel removed from the site. Rescue workers covered the 36-foot column with inscriptions, prayers, and signatures through the nine-month recovery, until the steel itself became a message. It was carried out on May 30, 2002, in a ceremony marking the official end of the recovery. Nearby, the Survivors' Stairs, saved from demolition, led hundreds of fleeing workers to safety on the day of the attacks. Stand in front of the column and read what people wrote; the signatures are the museum's most honest exhibit.
The names, and the first attack
The memorial above remembers 2,983 people: 2,977 killed on September 11, 2001, and six killed in 1993, when a truck bomb in the parking garage became the first attack on the towers. The names are arranged by meaningful adjacencies — about 1,200 families asked for loved ones to be listed beside the people they worked with or died with, and ten pregnant women have "and her unborn child" after their names. Inside the museum, Spencer Finch's wall of 2,983 sky-colored panels gives every person a panel of its own. Walk the pools at dusk, pick one name, and read the names around it — the museum will still be there when you are done.