​​The Hall of Mosses, located in the Hoh Rain Forest within Olympic National Park on Washington state’s Olympic Peninsula, winds through an old-growth grove of maple trees covered with a thick layer of green moss. The Hoh Rain Forest is a temperate rainforest that receives an average of 140 inches of precipitation per year. The lush forest includes a thick canopy shading the undergrowth of ferns and mosses, creating a bright green landscape everywhere you look.

While Yellowstone may be the oldest U.S. national park and Wrangell-St. Elias in Alaska may be the largest, neither can match the biodiversity of Great Smoky Mountains National Park. During the last Ice Age, many species migrated to the Great Smoky Mountains, which is one of the world’s oldest mountain ranges. The variety and number of living organisms — from plants and animals to fungi — is greater than any other U.S. national park.

As the second-oldest capital city in the Western Hemisphere, founded in 1521, it's not surprising that San Juan, Puerto Rico, has plenty of historical buildings. One of these is La Fortaleza, the oldest continually occupied governor's mansion in the Western Hemisphere. It was built in 1540 and has housed more than 170 governors of Puerto Rico since. It is part of UNESCO’s La Fortaleza and San Juan National Historic Site, recognized in 1983.

The Yosemite Firefall phenomenon occurs for about two weeks in February — and only if conditions are exactly right. Snowpack in the surrounding mountains melts and feeds Horsetail Fall, which flows over the edge of Yosemite’s famous granite monolith El Capitan. On clear evenings, the setting sun shines on the water and its mist, reflecting vibrant orange and red hues so that it looks like fire cascading down the mountain.

UNESCO heritage site and national park, Yellowstone is home to a whopping 10,000-plus hydrothermal features, including 500 geysers — which scientists estimate is about half of the world's geysers. The most famous is Old Faithful, which erupts around 17 times a day. Other breathtaking features, like the Beehive Geyser and Grotto Geyser, also provide a thrilling show of geothermal action.

As of 2023, the United States is home to 24 of the 1,100-plus UNESCO World Heritage Sites across the globe. These range from sites important to Indigenous history, such as Taos Pueblo, to sites of outstanding natural value, including several national parks. While its list is impressive, the U.S. has less than half as many UNESCO sites as Italy’s 58 — the most of any country.

Cahokia, the largest pre-Columbian settlement north of Mexico, was located near present-day St. Louis. Once home to as many as 20,000 people at its peak between 1050 and 1150, the city declined around 1400 and was slowly abandoned for reasons that remain a mystery. Left behind were about 120 earthen mounds, the tallest of which is Monks Mound, the largest prehistoric earthwork in the Americas, at nearly 100 feet tall.

The San Antonio Missions National Park encompasses five frontier missions along the river that shares its name with the Texan city. All were built in the 18th century as part of Spanish efforts to colonize the region’s peoples and convert them to Catholicism. Each mission was designed to be a self-sustaining community. As such, they included farmland, mills, and workshops in addition to living and worship spaces.

Thanks to his namesake tower in Paris, Alexandre-Gustave Eiffel was one of France's leading structural engineers in the late 19th century. After Frédéric Bartholdi, the original designer of the Statue of Liberty, unexpectedly died, he was a natural choice to finish the monument. Eiffel came up with the idea of a central spine in the statue, which functions as a connector for the various asymmetrical metal girders that give the statue its shape.

The list of UNESCO World Heritage Sites includes several sites important to U.S. history, including the Statue of Liberty (which joined in 1984) and Philadelphia’s Independence Hall (added in 1979), where both the Declaration of Independence and the U.S. Constitution were signed. Yosemite is one of several national parks on the list. As of yet, St. Louis’ Gateway Arch has not been named a UNESCO site.

The oldest, continuously lived-in housing in the U.S. is the Taos Pueblo in New Mexico. The pueblo has existed for almost 1,000 years, and the main structures — built entirely of adobe — were most likely constructed between 1000 and 1450. The pueblo is the only living Native American community that is designated as both a UNESCO World Heritage Site and a National Historic Landmark.

Named a UNESCO site in 1981, Mammoth Cave in west-central Kentucky has more than 412 miles of surveyed passageways. And there may be quite a lot more — scientists estimate the cave stretches about 1,000 total miles. The cave system is part of Mammoth Cave National Park, established in 1941 to protect the underground labyrinth of caves.

The Everglades National Park UNESCO World Heritage Site is home to the largest subtropical wetland ecosystem in North America, covering 1.5 million acres of central and southern Florida. Despite their preference for different habitats, you’ll find both alligators and crocodiles in the Everglades. American crocodiles are unique to the southernmost tip of Florida, which makes it the only place in the world where you can see these two semi-aquatic reptiles together in the wild.

The redwood is the world’s tallest tree, and the California redwoods are also some of the oldest living things on Earth. Redwood National and State Park, recognized as a UNESCO World Heritage Site, is a partnership between four parks in Humboldt and Del Norte counties, which contain nearly half of the world’s redwoods. The tallest individual tree, nicknamed Hyperion, was measured at 379 feet in 2006.

Frank Lloyd Wright’s visionary designs established him as one of the preeminent 20th-century American architects. The eight properties that share a UNESCO designation span 50 years of Wright’s career. They include the Unity Temple in Illinois and New York’s Guggenheim Museum, as well as houses in Chicago, Wisconsin, Los Angeles, Pennsylvania, and Arizona.

Monticello and the University of Virginia form a joint UNESCO World Heritage Site in Charlottesville, Virginia. The site was the home of founding father Thomas Jefferson, who was also an architect of neoclassical buildings and designed Monticello and an “academical village,” which is now part of the University of Virginia. According to UNESCO, the property symbolizes “the aspirations of the new American republic as the inheritor of European tradition.”

Alaska’s Wrangell-St. Elias and Glacier Bay National Parks combine with Canada’s Kluane National Park and Tatshenshini-Alsek Provincial Park to form a single UNESCO World Heritage Site, which covers almost 38,000 square miles. Alaska's Wrangell-St. Elias National Park is by far the United States' largest national park — at 13.2 million acres, it is almost six times the size of Yellowstone.

One of two UNESCO World Heritage Sites in Hawaii, Hawaii Volcanoes National Park contains two of the world’s most active volcanoes, Kilauea and Mauna Loa. The latter is the world’s largest active volcano. The level of volcanic activity has created an ever-changing landscape in the park. In addition to being a national park and a World Heritage Site, Hawaii Volcanoes is also an International Biosphere Reserve.

Situated in Philadelphia’s historic district, Independence Hall is known as the official birthplace of the nation. Fifty-six Revolutionary leaders gathered within its halls on July 4, 1776, to sign the Declaration of Independence. Eleven years later, Independence Hall also served as the gathering place for 12 state representatives to debate and sign the U.S. Constitution. Today, the historic building is a UNESCO World Heritage Site and is open to the public via ticketed tours.

Tied with California, New Mexico is home to three UNESCO sites: Carlsbad Caverns National Park, Taos Pueblo, and Chaco Culture National Historical Park. Carlsbad contains more than 100 limestone caves that feature unique speleothems inside. Taos Pueblo is an ancient pueblo village from the 13th and 14th centuries, while Chaco was a major Puebloan cultural center from the 10th to 12th centuries.

Today, there are more than 1,100 UNESCO World Heritage Sites that recognize places of outstanding cultural, historical, and natural value around the world. And it all started with a group of 12 places in 1978 — including two national parks in the United States. The country’s first two UNESCO sites were Yellowstone National Park (located in Wyoming, Idaho, and Montana) and Mesa Verde National Park in Colorado.