America's 20 most-visited National Monuments
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17. Organ Pipe Cactus National Monument, Ariz.( Joshua Bo / National Park Service )
Organ Pipe Cactus National Monument is named for the long, thin cacti that dot this desert landscape. But within the park visitors will find more than two dozen species of cactus and animals like the mountain lion and kangaroo rat that have adapted to the harsh habitat. This park is probably best visited in the winter when temperatures are in the 50s and 60s. In the summer temperatures often rise to over 100 degrees.
Visitors in 2009: 330,064
More info: http://www.nps.gov/orpi/index.htm