(CBS) — The Cook County Forest Preserve District just finished a year-long centennial celebration. A documentary, premiering at 7 p.m. Sunday on Channel 20, captures its evolving mission.

When landscape architects Dwight Perkins and Jens Jensen first proposed saving open space in Cook County early in the 20th Century, it was a radical idea that took several years to gain acceptance.

Over the years, its holdings have grown to nearly 70,000 acres.

Forest Preserve District spokesperson Lambrini Lukidis says at first, its value was mostly recreational. Now, she says, the environmental mission has gained equal importance and says it includes protection of flora and fauna found nowhere else in the United States.

The Forest Preserve District’s picnic groves, bridal paths and paths for runners and bicyclists remain immensely popular, but Lukidis says it offers courses and hands-on activities in many other areas dealing with plant and animal life and the environment.

The documentary captures its forests, prairies, wetlands, marshes and streams in all four seasons. In addition to frequent airings through September on Channel 20, the hour-long documentary is playing for free this month at Navy Pier’s IMAX theater.