Ask how many continents there are and the first answer is seven: Asia, Africa, North America, South America, Antarctica, Europe and Australia. But continents are defined by convention rather than by a strict rule, so people count them in different ways; the seven-continent model is the one most commonly taught in countries such as the United States. Some people count only six continents by joining North America and South America into one called America, and another six-continent model, mostly used in Russia, Eastern Europe and Japan, joins Europe and Asia into one called Eurasia. Sizes depend on definitions too. Asia covers about 44.6 million square kilometers, almost a third of the world's land. Africa is the second-largest and covers more than three times the area of the United States. Europe holds just 7 percent of the world's land. Australia, the smallest continent, covers just under 8.5 million square kilometers, but the country of Australia has a total area of 7,688,287 square kilometers, and the world's sixth-largest country by that measure. Together the continents add up to about 148 million square kilometers of land.

The Sahara shows how a boundary changes a number. Sources give its area as anywhere from 8.5 million to 9.4 million square kilometers, depending on how its edges are drawn. Either way it is the largest hot desert on Earth. It makes up about 25 percent of Africa. Dunes, called ergs, cover only 20 percent of it, and gravel plains called regs cover 70 percent. Only a small part of the world's deserts is the endless dunes people imagine, because desert is defined by little rain, not by sand. Rivers invite the same argument. The Nile flows over 6,800 kilometers before it empties into the Mediterranean Sea, and some researchers have contested the Nile's status as the world's longest river and credit the Amazon instead; the Amazon's measure is its volume: more water flows from it than from the next six largest rivers combined. NASA says the Amazon basin spans at least 6 million square kilometers, while National Geographic gives almost 7 million. About 80 percent of the basin is rainforest, and the rainforest is home to one-fifth of the world's land species.

Mountains are measured by people, too. Mount Everest soars to about 8,850 meters, according to National Geographic; the Survey of India gave about 8,848 meters in 1954, a National Geographic GPS team gave 8,850 meters in 1999, and a Chinese team gave 8,844.43 meters in 2005, which left out the snow cap. A Nepali team of surveyors reached the top of Everest in May 2019, and a Chinese team measured it in 2020. As of November 2026, the official height, announced jointly by Nepal and China in December 2020, is 8,848.86 meters. As of November 2026, the U.S. Geological Survey measurement of Denali, made in 2015, is 6,190 meters, making it North America's highest peak. As of November 2026, the depth of Lake Baikal in southern Russia is given as 1,642 meters, the deepest lake in the world. Europe's highest peak, Mount Elbrus at 5,642 meters, is in the Caucasus Mountains of Russia, and Aconcagua, the highest peak in the Andes, stands at 6,962 meters. NASA calls Kilimanjaro, at 5,895 meters, the tallest free-standing mountain.

Cold is measured with care as well. As of November 2026, the lowest air temperature ever measured on Earth is -89.2 degrees Celsius, at Vostok Station in Antarctica on 21 July 1983; the coldest place on Earth is on the East Antarctic Plateau, but not at the highest peak, and the coldest spots sit in hollows just downhill from a ridge. Temperatures in Antarctica, much lower than Arctic temperatures, plunge lower than -73 degrees Celsius. The ice itself is measured by area and by volume. The Antarctic Ice Sheet covers almost 14 million square kilometers and contains about 30 million cubic kilometers of ice. If all of it melted, sea level would rise about 58 meters. Together the Antarctic and Greenland ice sheets contain more than 99 percent of the land ice and over 68 percent of the fresh water on Earth. Scientists often discuss the Antarctic Ice Sheet as three regions: East Antarctica, West Antarctica and the Antarctic Peninsula.

Antarctica is the windiest, driest and iciest place on Earth. Penguins and seals make their homes there even though it is covered in ice. Captain Cook became the first recorded European navigator to cross the Antarctic Circle in January 1773, and Bellingshausen reported seeing an ice shore of extreme height in the last week of January 1820. Borchgrevink's 1899 expedition was the first to spend a winter on the Antarctic mainland, and Roald Amundsen was the first person to reach the South Pole, in December 1911. The Antarctic Treaty, signed in Washington on 1 December 1959 by twelve countries, says Antarctica shall be used for peaceful purposes. The Antarctic Treaty also guarantees freedom of scientific investigation in Antarctica and cooperation toward that end.

Another argument is about Zealandia. A research team led by New Zealand's geoscience agency GNS Science proposed in February 2017 that Zealandia, a 4.9 million square kilometer region of the southwest Pacific Ocean, is a continent. Roughly 94 percent of it is underwater. The team argued that its isolation from Australia and its large area support defining it as a continent, and it once made up approximately 5 percent of the area of the supercontinent Gondwana. Microcontinents like New Zealand lack cratons, which are areas of ancient crust formed during Earth's early history. Alfred Wegener proposed continental drift in the early 1900s, and it took several more decades for most scientists to accept it. By 240 million years ago almost all of the world's land was joined in a single huge continent, and the big supercontinent was called Pangaea. By about 200 million years ago it began to break apart. The positions of the continents are always changing.

Words to know

• convention: agreed habit
• cratons: ancient crust
• Zealandia: proposed continent
• volume: amount of space
• Gondwana: old supercontinent