Hippopotamus males fighting in Kruger National Park, South Africa (© Nigel Dennis/Gallo Images/Getty Images)

Aerial view of the coast along Lake Argentino, Glaciers National Park, Argentina (© Michele Molinari/Danita Delimont)

Talbot Lake in Athabasca River Valley, Jasper National Park, Alberta (© Tony Ernst)

Grand Canyon National Park, Arizona (© Jean-Pierre Lescourret/SuperStock)

Little Tahoma peak in the background with wildflowers in the Upper Paradise Valley of Mt. Rainier National Park, Washington (© Stephen Matera)

Crowfoot Mountain reflected in Bow Lake in Banff National Park in Alberta, Canada (© Daisy Gilardini/Getty Images)

Redwood trees in Redwood National Park, California (© William Manning/Corbis)

Racetrack Playa in Death Valley National Park, California (© Robert Harding World Imagery/Neale Clark/Getty Images)

Full moon over the Sonoran Desert in Saguaro National Park, Arizona (© Buddy Mays/Corbis)

Mt. Clements and Mt. Oberlin viewed from the Highline Trail with bear grass in bloom, Glacier National Park, Montana, U.S.A. (© Shannon Nace/Lonely Planet Images)

Natural arch in Hickman Bridge, Capitol Reef National Park, Utah (© Don White/Super Stock)

White-lined leaf frog walking on branch at Manu National Park, Cruz de Mayo, Madre de Dios, Peru (© Frans Lemmens/Getty Images)

Yellow Mound Overlook area of Badlands National Park, South Dakota (© Carol Barrington/Aurora Photos)

Infant gelada baboon in Simien Mountains National Park, Ethiopia (© Fiona Rogers/Corbis)

Snow gum trees in Cradle Mountain-Lake St Clair National Park, Tasmania, Australia (© Rob Blakers/Getty Images)

Snowcapped San Juan range above the autumn colors in Dallas Creek Valley, Uncompahgre National Forest, Colorado (© Terry Donnelly/Aurora Photos)

Deer Cave, Mulu Natioal Park, Sarawak, island of Borneo, Malaysia (© Richard Ashworth/Photolibrary)

A visitor near giant sequoias in Yosemite National Park, California (© Frans Lanting /Danita Delimont) ©

Small waterfall in Hamersley Gorge, Karijini National Park, Australia (© Alessandra Sarti/Photolibrary) ©

Cup fungi (Cookeina tricholoma) in Kinabalu National Park on the island of Borneo (© Frans Lanting/Corbis) ©