Ain't the WWW WWWonderful! I found the answer on this web site:
http://www.paintingsdirect.com/content/abt/093099/
dirt3.html
Walter de Maria - the quintessential 'earth artist!'
De Maria created the first of three Earth Room installations in 1968 at the Heiner Friedrich Gallery in Munich, Germany; this installation was one of his best-known works.
One version, The New York Earth Room, 1977, is permanently installed in a second-floor loft in New York City, owned by the Dia Center for the Arts.
Visitors climb stairs to confront a Plexiglas barrier holding back a gallery full of dirt 22 inches deep. They can look at but not walk over an expanse of rich loamy soil, moistened and picked clean of debris on a regular basis. There is no art object separate from the experience of viewing, and nothing to buy or sell, a circumstance that is typical of much conceptual art of the late 1960s and 1970s.