Papahanaumokuakea Marine

• At nearly 140,000 square miles, the Papahanaumokuakea Marine National Monument is the largest conservation area protected by the U.S. government. This natural monument is in the Pacific Ocean, next to the Hawaiian Islands. According the monument's official website, more than 7,000 marine wildlife species live in the Papahanaumokuakea Marine National Monument; among these species are the Hawaiian monk seal and green sea turtle. Papahanaumokuakea Marine was declared a national monument by the U.S. government in 2006.
  
  

Painted Desert

• Located in northeast Arizona, the Painted Desert is a 93,000-acre natural landmark that stretches from the Grand Canyon to the Petrified Forest in central Arizona. As its name suggests, the Painted Desert is named after the variety of its rocks' colors, which include pink, orange and red. The rock's colors are the result of volcanic activity and the shifting of tectonic plates over millions of years. The Painted Desert is accessible from its eastern end, home of the Painted Desert Visitor's Center, and western end, which is near Flagstaff.
  
  

Fossil Butte

• One of the Wyoming's newest U.S. National Monuments is Fossil Butte, a rocky formation in western region of the state; the monument was added to the National Monument list in 1972. This natural monument area is more than 8,000 acres and is significant because of its high concentration of freshwater fish fossils, including fossils of paddlefish and garpike. Fossil Butte also features the fossil of a 13-foot crocodile. Two hiking trails are available for visitors.
  
  

Grand Canyon

• Grand Canyon National Park in northern Arizona and southern Utah is home of the Grand Canyon, a 277-mile long gorge surrounding the Colorado River. At certain spots, this natural landmark is 18 miles wide and a mile deep. The Grand Canyon became one of the first protected national parks in the United States at the behest of President Theodore Roosevelt. Visitors may view the Grand Canyon from the canyon's South Rim or North Rim. The ancient Pueblo Native Americans dwelt in the caves within the Grand Canyon.
  
  

Mammoth Cave

• Mammoth Cave is in central Kentucky's Cave Country region and protected by the U.S. government in Mammoth Cave National Park. The Mammoth Cave tunnel system is the one of longest of its kind in the world with more than 350 miles of underground cave passages. This cave is made up of limestone and sandstone layers. The National Park Service provides guided tours in several sections of Mammoth Cave.