The largest animal that has ever lived, blue whales (Balaenoptera musculus) can grow to 30 meters (100 feet) long, which is longer than the length of a basketball court. They can weigh as much as 203,210 kilograms (200 tons), about the weight of 15 school buses.

So when a blue whale does an ordinary act that is necessary for its own survival—even just breathing or gliding through the ocean—it can be an awe-inspiring event.

“I think it’s humbling,” says National Geographic Explorer John Calambokidis, a marine mammal research biologist who studies blue whales. “Often, we are working in boats 18 feet [5.5 meters] long, and these animals we are approaching can be five times larger.”

Calambokidis is co-founder of Cascadia Research, a nonprofit organization based in the U.S. state of Washington that studies marine mammals in an attempt to help protect them. Calambokidis has traveled all over the Pacific Ocean—from off the coast of Costa Rica, where he was funded by expedition grants from the National Geographic Society, to the Queen Charlotte Islands in British Columbia, Canada. He travels to research blue whales. Having observed about 5,000 blue whales in the wild, Calambokidis has witnessed the behaviors of the enormous creatures up close and personal.

The scientist can easily recall the first time he heard the geyser-like explosion of air from a blue whale breathing nearby.

“Here’s this massive animal exhaling virtually all of the air in its lungs in a fraction of a second,” he says. “It comes out with an explosive force. It literally made me jump in the boat.”

Calambokidis says a blue whale’s call, which the animal uses to communicate with other members of its pod and possibly to sonar-navigate the oceans, is a far cry from the sounds emitted by other marine organisms.

“The calls they make underwater are some of the loudest sounds any animal makes,” he says.

But the low-frequency sound is normally below a human’s hearing range. Calambokidis never heard a blue whale’s call during the first decade he was studying the animal. Eventually, with the help of a hydrophone, a microphone that picks up noises underwater, he was able to hear the deep, pulsating sound.

“Hearing that reverberate through was one of the more impressive times I’ve heard an animal call,” he says.

Just as impressive as its call is the noticeable mark a blue whale leaves behind as it sinks into the sea. The phenomenon is called a flukeprint.

“The flukeprint is created by the upward motion of water coming off the trailing edge of the fluke [the whale’s tail] as the animal basically begins its acceleration downward,” Calambokidis says. “So what it ends up looking like is basically an upwelling mass of water that starts as a small circle and spreads outward.”

There is one striking aspect of a blue whale that has nothing to do with the animal’s enormous size—its appearance underwater.

“Especially in the sunlight, when they are traveling just below the surface, they can get this almost shimmering light, an almost turquoise glow,” Calambokidis says. “They can almost seem to glow underwater.”

While blue whales dwarf their adversaries in the sea, the large marine mammals are surprisingly timid, Calambokidis says.

“Their response to killer whales is to flee at high speed,” he says. “People might not expect that from the largest animal that has ever lived.”

Though a blue whale’s tongue can weigh as much as an elephant, the giant ocean creature survives almost exclusively on a diet of two-inch shrimp-like organisms called krill. During certain periods of the year, a blue whale can eat as much as 4,064 kilograms (4 tons) of krill in a single day. It then expels the processed food in a defecation trail, which Calambokidis describes as a brick-red cloud that colors the water.

The world population of the docile creatures dwindled severely from 1900 to the mid-1960s, when blue whales were being hunted extensively for whale oil. Whale oil is a substance made from whale’s fat, or blubber. Whale oil can be used as a heating and lighting fluid. It is estimated that 360,000 blue whales were killed during that period. In 1966, the killing of blue whales was banned by the International Whaling Commission.

Scientists believe there are between 10,000 and 25,000 blue whales left in Earth’s oceans.

Even though whale hunting no longer threatens the endangered species, Calambokidis says there are other dangers facing the blue whale today. Underwater sounds from ships and sonar might affect the animals. Many scientists worry that climate change could alter the whales’ ecosystems. One definite cause of blue whale deaths occurs when large ocean vessels inadvertently strike the marine mammals. In fall 2007, four blue whales were killed by ships off the coast of Southern California.

With Cascadia Research, Calambokidis is trying to learn more about blue whales in an effort to ensure their population doesn’t dwindle any more.

“They do represent one of the more magnificent animals that we have on the planet,” he says.