From ArborWiki

Ann Arbor's Allen Creek is extraordinarily well-known for a body of water that is completely underground (though it spawns parallel surface streams during heavy rains). The underground drain that is the main branch of the creek runs from the vicinity of Michigan Stadium, which pumps groundwater into the drain to prevent flooding on the field, north along the Ann Arbor Railroad's tracks to an outlet just below Argo Dam. Several tributaries flow into the main drain from the west, roughly paralleling Liberty, Huron, and Miller Streets. One of these branches is easily traceable through West Park in any wet weather.

Public awareness of the Creek's existence rose in 2004-2005, when the Downtown Development Authority's "Three Site Plan" envisioned construction in or near the Creek's floodplain, on the City-owned properties at the First & Washington Parking Lot, 415 West Washington, and 721 North Main Street. Protest of these plans by nearby residents coalesced as the "Friends of the Greenway" organization, which counter-proposed that these properties should be turned into parkland along a greenway based around a "daylighted" (brought back to the surface) Allen Creek.

Sources:

• Ann Arbor District Library, Map of Allen's Creek
• Sierra Club, Huron Valley Group, The Ann Arbor Greenway
• Ann Arbor is Overrated, Update on the Leopold Charrette
• Michigan Daily Viewpoint: Keep Ann Arbor Weird