CHICAGO (CBS) — On Monday, the former main Chicago Post Office will officially reopen. On Saturday, some architecture buffs got a peek inside the extravagant lobby, as part of the “Open House Chicago” event.

Visitors got rare access inside more than 300 buildings all across the city as part of the annual event.

The former post office, built in the 1920s, is undergoing an $800 million renovation.

It’s being transformed into nearly 3 million square feet of office space. One of the big-name tenants will be Uber, which signed a 10-year lease in the building in August. The company plans to move into the space in January 2020.