Image by Tom Mareschal Getty Images

A favorite hunting ground for runners, personal trainers and artistically tattooed sunbathers, scenic Green Lake Park surrounds a small natural lake created by a glacier during the last ice age. Two paths wind around the lake, but these aren’t enough to fill the needs of the hundreds of joggers, power-walkers, cyclists and in-line skaters who throng here daily; the city government now regulates traffic on the paths.

In the early 1900s, city planners lowered the lake’s water level by 7ft, increasing the shoreline to preserve parkland around the lake. After the lowering, however, Ravenna Creek, which once fed the lake, no longer flowed through. Green Lake became stagnant and filled with stinky green algae. Massive dredging efforts to keep it navigable continue, although the lake remains prone to algae blooms.