A landfill at Northwestern University that extended the university's campus out into Lake Michigan in the late 60's. Although half the lakefill is now used for parking, libraries, and scary biomedical research centers, the rest of it is a very pleasant park that runs about a half-mile down the lake. Highlights include soccer fields, a jogging path, and a 5-acre lagoon circled by the park section.
The lakefill is mostly sand, with a layer of topsoil. Erosion from the lake was prevented by making the lakefill shore out of huge stone blocks (think 4 feet on a side) instead of a beach or nothing at all. Students will often write declarations of love to each other on the sides of the blocks; seeing "Jessica, I love you! Ramone 1997" as you walk by and realizing that at one point, long ago, those two were happy together, is quite uplifting.
The park part is what students at Northwestern most often mean when they refer to "The Lakefill". Chilling on the lakefill is one of the best things to do on a spring afternoon while skipping class.