Given, The Waterfall, The Illuminating Gas.....

Étant donnés by Marcel Duchamp, 1946-66. Mixed media assemblage: (exterior) wooden door, iron nails, bricks, and stucco; (interior) bricks, velvet, wood, parchment over an armature of lead, steel, brass, synthetic putties and adhesives, aluminum sheet, welded steel-wire screen, and wood; Peg-Board, hair, oil paint, plastic, steel binder clips, plastic clothespins, twigs, leaves, glass, plywood, brass piano hinge, nails, screws, cotton, collotype prints, acrylic varnish, chalk, graphite, paper, cardboard, tape, pen ink, electric light fixtures, gas lamp (Bec Auer type), foam rubber, cork, electric motor, cookie tin, and linoleum. 95 ½ x 70 x 49 inches. The Philadelphia Museum of Art, Philadelphia, PA.

From the Philiadelphia Museum of Art’s website:
No photograph can communicate the intensity of the unique visual experience of seeing Marcel Duchamp’s Étant donnés: 1. La chute d’eau, 2. Le gaz d’éclairage (Given: 1. The Waterfall, 2. The Illuminating Gas), which the artist constructed in total secrecy over a twenty-year period, from 1946 to 1966. The unsuspecting viewer encounters a spectacular sight: a realistically constructed simulacrum of a naked woman lying spread-eagle on a bed of dead twigs and fallen leaves. In her left hand, this life-size mannequin holds aloft an old-fashioned illuminated gas lamp of the Bec Auer type, while behind her, in the far distance, a lush wooded landscape rises toward the horizon. This brightly illuminated backdrop consists of a retouched collotype collage of a hilly landscape with a dense cluster of trees outlined against a hazy turquoise sky, replete with fluffy cotton clouds. The only movement in the otherwise eerily still grotto is a sparkling waterfall, powered by an unseen motor, which pours into a mist-laden lake on the right.  As Surrealism recast itself in the 1940s in reaction to the rise of fascism and the carnage of World War II, its protagonists increasingly turned to an interior world, such as the one seen behind the massive Spanish wooden door in Étant donnés, which separates the viewer from an unexpected and unimaginable landscape, visible only by looking through the peepholes. It is only in the last 15 years that photos were allowed to be taken of this piece. Duchamp constructed it in secret in a hotel room in New York, He publicly stated that he had given up art. After he died, the existence was revealed in his will along with the keys to the hotel room and the instructions for the assemblage which were in a safety deposit box in a NYC Bank. The piece was given to the Philadelphia Museum of Art under the condition that no photographs were to be allowed for 20 years after it's installation. The only access was through the crack in the massive wooded door he specified.