Mudbrick Houses, Shibam, Yemen, 2005

Photograph by George SteinmetzThe city of Shibam, Yemen, rises from the Rub al Khali, or Empty Quarter, a sea of sand that occupies one-fifth of the Arabian Peninsula. The distinctive mud-brick skyscrapers that house Shibam's 5,000 residents have earned the city its nickname: "Manhattan of the Desert."

Cars Passing the Colosseum, Rome, Italy, 1981

Photograph by O. Louis MazzatentaNearly 2,000 years ago, the Colosseum was built to host gladiator duels, battle reenactments, and other public spectacles. Now, the 50,000-seat stone-and-concrete amphitheater serves Rome in another capacity: as a traffic circle.

Women Carrying Firewood, Sudan, 2003

Photograph by Randy OlsonWomen carry bundles of firewood on their heads in the Sudanese desert. In rural African cultures, the responsibility of foraging for firewood usually falls to the women. In Sudan's brutally dry desert, where little vegetation grows, it can take several hours each day to collect enough wood to cook with.

Cloudy Sky, Location Unknown, 2004

Photograph by Peter Essick

Piles of cotton-like cumulus clouds fill the sky on a clear day. Mid-level cumulus clouds form between 6,500 feet (2,000 meters) and about 20,000 feet (6,000 meters). They form when humid air cools enough for water vapor to condense into droplets or ice crystals. A single cloud can hold billions of pounds of water, but may not always produce rain.Earth Day, celebrated annually on April 22, marks the anniversary of the 1970 birth of the environmental movement. Scientists warn that rising temperatures worldwide could fuel extreme weather—just one of many damaging effects of global warming.

Byzantine Coin, Sinop, Turkey, 2001

Photograph by Randy Olson

Explorers pulled this ancient Byzantine coin from a Black Sea wreck near Sinop, Turkey. Millennia ago, the Black Sea was a freshwater lake. When the last ice age waned some 12,000 years ago, salt water from the Mediterranean breached the Bosporus Valley, transforming the Black Sea into its current state—a toxic, brackish, oxygen-deficient pool, perfect for preserving ancient wrecks.

Tuamotu Archipelago, French Polynesia, 1997

Photograph by David Doubilet

A school of fish swims over a coral formation in the Tuamotu Archipelago of French Polynesia. Like a few grains of rice sprinkled on a sea of blue, the French territory in the South Pacific is a paltry 1,359 square miles (3,520 square kilometers) of land scattered over one million square miles (2.6 million square kilometers) of ocean, an area as large as Western Europe.

(Photo shot on assignment for, but not published in, "Black Pearls of French Polynesia," June 1997, National Geographic magazine)