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CHICAGO —What was once an airport on Chicago’s lakefront is now a park featuring nature trails, rolling hills, migratory birds and native plantings.

The 40-acre expanse of green space officially opened Friday on the southern half of Northerly Island, which hosted the 1933 World’s Fair before it became an airport.

It’s been 12 years since then-Chicago Mayor Richard M. Daley, citing security concerns following the 9/11 terror attacks, sent backhoes to carve X’s into the Meigs Field runway.

The city spent $9.7 million to build the park, which features man-made hills, a 5-acre lagoon and a winding bike and pedestrian trail. There are also camping sites and areas for educational programming.

Northerly Island is actually a peninsula linked by a land bridge that leads to the city’s Museum Campus.