National September 11 Memorial & Museum, New York City (© Antonino Bartuccio/Sime/eStock Photo)

River Thames, London, England (© Puthipong Worasaran/Getty Images)

'Power of the Titans' mural by Andrei Krautsou, North Kansas City, Missouri (© Stephen Taylor/Alamy)

Crystal Lake in the Enchantments, Alpine Lakes Wilderness, Washington (© Mitch Pittman/Tandem Stills + Motion)

Dusky eagle-owls, Pakistan (© zahoor salmi/Getty Images)

Toronto city skyline seen from Lake Ontario, Canada (© Roberto Machado Noa/Getty Images)

Ancient beech tree, Glenariff Forest Park, County Antrim, Northern Ireland (© Dawid K Photography/Shutterstock)

Anna's hummingbird, Santa Cruz, California (© yhelfman/Getty Images)

Interior of the Stockholm Public Library, Sweden (© Andrei Hrabun/Alamy)

Iguazu Falls at the border of Argentina and Brazil (© AirPano LLC/Amazing Aerial Agency)

Vasco da Gama Bridge, Lisbon, Portugal (© Achim Thomae/Getty Images)

Dolphin pod near Réunion, overseas department of France, Indian Ocean (© Joost van Uffelen/Shutterstock)

Lighthouse at Point Reyes National Seashore, California (© RMB Images/Photography by Robert Bowman/Getty Images)

Sunrise at Ahu Tongariki, Rapa Nui National Park, Easter Island, Chile (© Piriya Photography/Getty Images)

Casa del Prado, Balboa Park, San Diego, California (© f11photo/Getty Images)

Wallabies at sunrise, Adelaide River, Northern Territory, Australia (© Jeremy Woodhouse/Getty Images)

Mid-Autumn Festival in Gardens by the Bay, Singapore (© Victor Y. Zhuo/Alamy)

Gujō Hachiman Castle, Gifu prefecture, Japan (© ta2funk ito/500px/Getty Images)

Playground at the Obereversand Lighthouse in Dorum-Neufeld, Germany (© INGO WAGNER/DPA/AFP via Getty Images)

Ocracoke Lighthouse on Ocracoke Island, North Carolina (© Chansak Joe/Getty Images)

Oktoberfest in Munich, Germany, at sunset (© AllesSuper21/iStock/Getty Images)

Buttermere, Lake District National Park, England (© Alan Novelli/Alamy)

Sea otter, Prince William Sound, Alaska (© Gerald Corsi/Getty Images)

Waterfall in Skaftafell, Vatnajökull National Park, Iceland (© Nopasorn Kowathanakul/Getty Images)

Giant sequoias, Sequoia National Park, California (© Galyna Andrushko/Shutterstock)

Blue-throated toucanet, Los Quetzales National Park, Costa Rica (© Oscar Dominguez/Tandem Stills + Motion)

Aerial view of the Grand Canal and the Basilica di Santa Maria della Salute, Venice, Italy (© Bachir Moukarzel/Amazing Aerial Agency)

Cottonwoods seen through an arch in Coyote Gulch, Glen Canyon Recreation Area, Utah (© Stephen Matera/Tandem Stills + Motion)

West Cornwall Covered Bridge over the Housatonic River, Connecticut (© pabradyphoto/Getty Images)

Herd of walruses in northern Spitsbergen, Svalbard archipelago, Norway (© AWL Images/Danita Delimont)