J. GERARD SIDANER / SCIENCE PHOTO LIBRARY J. GERARD SIDANER / SCIENCE PHOTO LIBRARY
Boats weed out duckweed, a blight in the canals of the Xochimilco Floating Gardens, an ecological site in Mexico. The canals, also called chinampas, were once used for farming purposes. Now the area has been engulfed by Mexico City and is primarily a tourist attraction. UNESCO designated it a World Heritage Site in 1986.
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