The largest ancient castle in the world is Prague Castle in Prague. Built in the ninth century, it is an oblong irregular polygon with an average diameter of 420 feet and a total surface area of 18 acres. At times, it has housed the kings of Bohemia as well as Holy Roman emperors and presidents of Czechoslovakia. After Czechoslovakia split into the Czech Republic and Slovakia in 1993, the castle became the seat of the head of state of the new Czech Republic.

Blue whales, on average, weigh 176 tons. Their tongues alone are as heavy as an elephant. They reach these behemoth proportions on a diet of tiny shrimplike animals called krill. Blue whales were hunted almost to extinction but are now on the International Union for Conservation of Nature’s list of endangered species. They live in all the world’s oceans, but travelers who wish to spot these rare creatures might try a whale-watching cruise in the Gulf of Maine, which runs from Massachusetts to Nova Scotia. Visitors to Santa Catalina Island, Calif., can try to see them from December to March. Car ferries crossing the Bay of Biscay (north of Spain and west of France) often pass these animals, and Iceland also offers blue-whale-watching tours.

Dubai, United Arab Emirates, opened the world’s tallest skyscraper in January, and superlatives have poured in ever since. If you stuck the Eiffel Tower on top of the Empire State Building, you still wouldn’t have a structure as tall as the Burj Khalifa. It rises 2,717 feet from the desert and provides views of the Persian Gulf, the sail-shaped Burj al-Arab hotel and the manmade Palm Jumeirah island. Originally named Burj Dubai, the building was renamed in honor of Sheik Khalifa bin Zayed Al Nahyan, the ruler of Abu Dhabi, which pumped tens of billions of dollars into Dubai last year as it struggled to pay enormous debts. Burj Khalifa also plans to have the world's highest mosque and swimming pool.

Royal Caribbean’s Oasis of the Seas debuted in December as the world’s largest cruise ship. It’s nearly five times the gross tonnage of the Titanic, 1½ times longer than the U.S. Capitol building, has 16 decks and has a capacity for 6,296 guests. At 1,187 feet long, 208 feet wide and 213 feet in air draft, it’s too long, too wide and too tall to fit through the Panama Canal.

The Great Barrier Reef off Queensland, Australia, is so large that it can be seen from space. It is 1,260 miles long and home to thousands of separate reefs. It’s a breeding area for humpback whales as well as six species of sea turtles, including the endangered green sea turtle. About 125 species of shark, stingrays, skates and chimera live on the reef. Swimmers, snorkelers and divers beware: It’s home to dangerous creatures, too, including the box jellyfish (which can inflict a fatal sting on humans), the blue-ringed octopus (whose venom can kill an adult human in minutes) and 15 species of sea snakes (all of which are lethal).

Abu Dhabi, United Arab Emirates, just became home to the world's largest LED project. The Yas Hotel looks like something out of a science fiction movie or, in the words of the hotel, “a giant fishing net tossed over the sea.” The eye-catching hotel is wrapped in 5,000 LED lights that can change colors and even display video. The hotel has several restaurants, a spa, two rooftop swimming pools and access to an 18-hole golf course on Yas Island.

The world’s largest swimming pool is the San Alfonso del Mar seawater pool in Algarrobo, Chile. It is 3,324 feet long, longer than 20 Olympic-sized swimming pools laid end to end. Water from the neighboring Pacific Ocean is heated to a comfortable 79 degrees Fahrenheit for swimming. Sports enthusiasts can sign up for classes in swimming, sailing, kayaking and scuba diving.

Trees invite superlatives. The world’s biggest tree is General Sherman, a giant sequoia that’s 275 feet high with a trunk volume of about 1,487 cubic meters. You’ll find it in Giant Forest within Sequoia National Park east of Visalia, Calif. General Sherman is believed to be between 2,300 and 2,700 years old.

Mount Everest, 29,029 feet, is the world’s highest mountain. In 1953, New Zealand’s Edmund Hillary and Nepal’s Tenzing Norgay were the first to reach the summit. Mount Everest has claimed the lives of 133 mountain climbers, including eight who died in the 1996 Everest disaster documented in Jon Krakauer’s book “Into Thin Air.” The oldest person to climb Mount Everest was Min Bahadur Sherchan of Nepal, who summited at the age of 76 years and 340 days.

Georgia Aquarium in Atlanta, which opened in 2005, is the world’s largest, with 8 million gallons of water. It contains the largest collection of giant groupers, potato groupers, humphead wrasses, tarpons, giant trevallies, batfish, sawfish, blacktip reef sharks, giant hammerhead sharks and wobbegong sharks in an aquarium.  It’s also the only aquarium outside of Asia to house whale sharks, the world’s largest fish. You can swim or scuba dive with these whale sharks as part of the aquarium’s programs. The swim program allows participants to swim at the surface with an air supply, which does not require any certification.

Northeast Greenland National Park is the largest national park in the world, with an area of 375,000 square miles. It is therefore larger than 163 countries. It’s the only national park in Greenland, as well as the most northerly national park in the world. Visitors will find virtually no humans but can look for polar bears, musk oxen, walruses and five species of seals. Narwhals migrate into the fjords in summer. The park is north of the Arctic Circle, so the aurora borealis can be observed almost year-round, but the best times are from October to early April.

Kingda Ka roller coaster at Six Flags Great Adventure in Jackson, N.J., is the tallest and fastest roller coaster in the world. The train climbs to 456 feet and can accelerate to 128 mph in 3.5 seconds. The thrill ride travels 3,118 feet and lasts 59 seconds.

Angel Falls in Canaima National Park, Venezuela, is the highest waterfall in the world. It has a total drop of 3,212 feet, which makes it higher than Burj Khalifa, the world’s tallest skyscraper (which is 2,717 feet). Angel Falls was named after the U.S. pilot Jimmie Angel, who recorded it in his log book in 1933. If you’re wondering where to find the largest waterfall in the world, head to Boyoma Falls in the Democratic Republic of the Congo. It flows at a rate of 600,000 feet per second.

The Amazon rain forest encompasses 1.7 billion acres and stretches across Brazil, Peru, Colombia, Venezuela, Ecuador, Bolivia, Guyana, Suriname and French Guiana. It is, however, shrinking at an alarming rate. National Geographic magazine reports that close to 20 percent of the Amazon has been cut down in the last 40 years. The Amazon has the world’s richest diversity of birds, freshwater fish and butterflies and is home to jaguars, pink dolphins, howler monkeys, southern two-toed sloths, poison dart frogs and scarlet macaws.

The Bawabet Dimashq, also known as the Damascus Gate restaurant, in Damascus, Syria, is the largest in the world, with 6,014 seats. What’s on the menu? Just about everything. The huge restaurant complex has six culinary themed sections for Indian, Chinese, Arab, Iranian, Middle Eastern and Syrian cuisine. Local specialties include lamb with thyme, steak with yogurt and beans, tabbouleh, hummus and baba ghanoush. During busy summer months, the restaurant employs up to 1,800 people in the dining area and kitchen.