Following a decade of development, The National September 11 Memorial Museum will open to the public on May 21. On top, it's an inviting, transparent pavilion. Below, it's a dark time capsule full of haunting artifacts.

Designed by architecture firm Snøhetta, the pavilion appears as an airy, asymmetrical mix of metal and glass. The exterior contains mirrored stripes that reflect the city, and the windows open to the skyline, wrapping visitors in NYC itself.

Clothing from Chelsea Jeans remains covered in toxic rubble. Photo by Jin Lee.

Inside, visitors see two massive structural columns from the original towers in an otherwise neutral space defined by its vaulting windows that bathe the space in light. It's meant to invoke a warm and inviting feeling, that is, until they descend below ground into the museum, designed by architecture firm Davis Brody Bond.

Here, only a few rays of sun peek through. Under the relatively dim glow of bulbs, visitors can explore artifacts from 9/11 frozen in time–including a Herculean grappler still excavating the site, clothing racks from Chelsea Jeans–a store just a block from the WTC–that remain covered in toxic rubble, shards of a Boeing 767, and a half-wrecked fire truck from Ladder 3, the FDNY fire company that suffered some of the greatest casualties in the rescue operation. The crew was last seen on the 40th floor of the North Tower before it collapsed.

Artifacts aren't the only way the museum pays tribute to the event. Local Projects has painted 34 feet of concrete with a digital portrait of 9/11 gathered from two million news articles. It must be a powerful feeling, to walk down the stairs into the museum–right alongside the –and arrive 13 years in the past.