World Oceans Day is Sunday, designated by the United Nations as a time to celebrate the extraordinary diversity of life beneath the waves and to focus on the increasing challenges the sea faces, from acidification to pollution and overfishing.

Over the last century, underwater photography has become one of the most important ways for people to experience and learn about the ocean.

The technology for documenting the deep has made huge strides, with remote-controlled submersible cameras revealing previously inaccessible depths and crittercams affixed to whales, seals, and sharks providing new windows into animal behavior. (See "The Evolution of Alvin.")

Digital cameras, meanwhile, have ended the days of diving with many cameras and endlessly resurfacing to change film.

"I used to take ten cameras into the field," says acclaimed underwater photographer David Doubilet. "Make that ten cameras, 20 strobes, 12 cases of equipment. The best you'd get out of ten cameras was 350 pictures before you had to surface and reload."

A digital camera can hold thousands of images on a single memory card.

But even with the increasing sophistication of the equipment, the photographer's skill is still paramount. Underwater photography presents challenges that no land-based photographer need contend with. Just consider the subject matter.

"The first thing a fish wants to do is not be photographed," says Doubilet. "At some point the fish has to look at you. The fish has to be doing something. You can't pay a fish to pose. You can't act like a paparazzi cornering a fish in a nightclub."

View Images PHOTOGRAPH BY BILL CURTSINGER, NATIONAL GEOGRAPHIC

This 1968 photo reveals a leopard seal peering through a veil of plankton. (See "How a Leopard Seal Fed Me Penguins.")

Although the technology has improved since that image was made, underwater photography remains an equipment-intensive discipline.

"It requires all the equipment used by land photographers, plus so much more," says underwater photographer Brian Skerry. A photographer sets out for the field, he says, with up to 30 cases full of underwater housings, special strobes, not to mention the diving gear—wetsuits, dry suits, masks snorkels, fins, regulators buoyancy compensators.

"I sometimes envy my street shooting colleagues who travel with only two or three camera bodies and a handful of lenses," Skerry says. "But then, they don't get to spend months with sharks or sea turtles."

View Images PHOTOGRAPH BY DAVID DOUBILET, NATIONAL GEOGRAPHIC CREATIVE

David Doubilet made this image in 1987 in the Bismarck Sea, off Hanover Island in Papua New Guinea. It shows a scientist ringed by barracuda. Although the toothy fish can appear menacing, scientists say they aren't aggressive to divers.

But there are plenty of other dangers.

Photographer and filmmaker Wes Skiles, who took breathtaking photographs of underwater caves, died in 2010 while diving off the Florida coast.

View Images PHOTOGRAPH BY PAUL NICKLEN, NATIONAL GEOGRAPHIC CREATIVE

Paul Nicklen, who specializes in photographing marine life in polar waters, made this striking image of a submerged polar bear in 2004.

Nicklen tells National Geographic this about working in the extreme temperatures of icy seas:

"You lose feeling in your lips, but you don't worry much about that. Within five minutes your hands get cold. After 15 minutes, your hands and feet are in pain. Then you lose all feeling in your hands, then in your feet. Your body shakes violently. After about 40 minutes, the shivering stops. Now you are getting into the danger zone. Your legs stop working. You haven't felt your fingers for half an hour, and have to check to see they are connecting with the shutter. That's when you think seriously about getting out. You are getting hypothermic and your body core temperature is dropping."

View Images Photograph by Brian Skerry, National Geographic Creative

Brian Skerry made this photograph off Tiger Beach in the Bahamas in 2005.

His work takes advantage of the ocean's changeability of light, vibrancy of color, and rich diversity of life.

Long-time National Geographic staffer Luis Marden made the first photo essay of color underwater photos in the February 1956 issue of the magazine.

"I seemed to hang suspended in the heart of an enormous liquid sapphire," Marden wrote of diving over a coral reef.

View Images Photograph by Brian Skerry, National Geographic Creative

This image by Brian Skerry reveals brilliant hues of sea pens and a blue cod in New Zealand's Fiordland National Park.

The deep-water dwelling sea pens were tricked into emerging at the shallower depth of 75 feet (23 meters) due to tannin-stained surface water that blocked out sunlight.

View Images Photograph by Brian Skerry, National Geographic Creative

This photograph by Brian Skerry from 2007 shows a close encounter between a diver and a southern right whale at a depth of 72 feet (22 meters), off New Zealand's Auckland Islands.

"Many of these southern rights in the Auckland Islands had never seen humans before in the water and were highly curious," Skerry wrote. "Swimming along the ocean bottom with a 14-meter (46-foot) long, 70-ton whale was the single most incredible animal encounter I have had."

View Images Photograph by Brian Skerry, National Geographic Creative

Brian Skerry made this image of shimmery fish off Muko-shima in the Bonin Islands, a tropical and subtropical chain about 620 miles (1,000 kilometers) south of Tokyo.

Lighting options for underwater photographers have improved since the time of Luis Marden in the mid 20th century. In those days large cameras were locked in watertight boxes. Flashbulbs often imploded under the pressure, gashing the unlucky photographer's hands. (Marden learned to wear chain mail gloves after suffering an embedded shard.)

View Images PHOTOGRAPH BY DAVID DOUBILET, NATIONAL GEOGRAPHIC CREATIVE

David Doubilet photographed this hawksbill sea turtle in 2009 in the Red Sea. Remora fish can be seen attached on the reptile's underside.

Scientists think remoras benefit from attaching to larger marine animals by receiving protection and access to leftover scraps of prey. They don't seem to hurt their hosts.

Some cultures in the Indian Ocean have long used remoras to hunt for turtles. They attach a line to the small fish and let it go. When the fish attaches to a turtle, the fishers reel both in.

View Images Photograph by Brian Skerry, National Geographic Creative

Brian Skerry made this photograph of a whale shark, the largest fish in the sea, swimming through a school of baitfish near Isla Holbox off Mexico's Yucátan Peninsula.

The area is one of the best places in the world to view whale sharks, which are relatively rare animals that sift small life forms from the sea for their sustenance.

View Images PHOTOGRAPH BY THOMAS P. PESCHAK, NATIONAL GEOGRAPHIC

This Cape fur seal surfs a wave in the Table Mountain National Park Marine Protected Area off the Western Cape of South Africa.

Cape fur seals are known to spend significant parts of their day in social and play behaviors, including "surfing" large waves.

View Images Photograph by David Doubilet, National Geographic Creative

These pink anemonefish were photographed in Kimbe Bay, Papua New Guinea.

Reef ecosystems are diverse and highly complex, with a wide range of animals taking up different niches. Some fish spend most of their lives in the protective stinging arms of an anemone, to which they are immune.

View Images Photograph by Brian Skerry, National Geographic Creative

A Southern Hemisphere species known as dusky dolphins were photographed in Argentina's Golfo Nuevo. The intelligent marine mammals work together to corral and feed on anchovies.

View Images PHOTOGRAPH BY DAVID DOUBILET, NATIONAL GEOGRAPHIC CREATIVE