One morning a few years ago, on a forested slope 6,200 feet above sea level in southwestern Tanzania, a team of wildlife researchers was tracking down reports about a strange primate. The scientists suspected that the animal, known to local hunters as kipunji, would turn out to be imaginary. Then someone yelled “Kipunji!” and everyone turned to gawk at what biologist Tim Davenport of the Wildlife Conservation Society described afterward as “the most bizarre monkey I had ever seen.” It was about three feet tall, with a thick fur coat and brownish-gray hair fanned out around its black muzzle like a Victorian gentleman’s cheek whiskers. “Bloody hell!” said Davenport. “That’s got to be a new species.”

It was of course astonishing for a large primate to be discovered in the 21st century in a heavily populated corner of East Africa, where human beings have been kicking around for as long as we have been human. (Scientists now know it as Rungwecebus kipunji—the monkey from around Mount Rungwe called kipunji—and think about 1,100 of the animals survive there.) But the truth is that big, colorful, even spectacular, new species seem to be turning up everywhere these days. We are living in what some naturalists have dubbed “a new age of discovery.” The number of species being found today “compares favorably with any time since the mid-1700s”—that is, since the beginning of scientific classification—according to Michael Donoghue of Yale University and William Alverson of Chicago’s Field Museum. These new species, they write, may be weird enough to induce the same “sense of awe, amusement, and even befuddlement that remarkable new organisms inspired during the last great age of discovery” from the 15th through the 19th centuries.

Conventional wisdom says such discoveries should not be happening now. But conventional wisdom always acts as if everything worth knowing is already known and as if all the good stuff has long since been discovered. The great French anatomist Georges Cuvier thought so as early as 1812, discounting the likelihood of “discovering new species of the larger quadrupeds” in the modern world. Then explorers discovered the gorilla, the okapi, the pygmy hippo, the giant panda and the Komodo dragon, among many others.

Nature, the scientific journal, pointed out in 1993 that although one might expect newfound species to be limited to “obscure microbes and insects,” scientists in Vietnam had just discovered a bovine. Then others discovered a striped rabbit in the Mekong Delta and a gaudy Indonesian fish that swims by bouncing haphazardly off the sea bottom.

Such novelties will turn up for years to come. Scientists estimate the total number of plant and animal species in the world at 10 million to 50 million—but they have so far described only about 1.9 million. (The standard definition of a species is a population of organisms that breed together over time and stay separate from other populations.) Even within our own class, mammals, roughly 300 new species have been discovered in the first decade of this century—mostly rodents, but also marsupials, a beaked whale and a slew of primates. Researchers recently estimated that the total mammal species count will rise from about 5,500 now to 7,500 by mid-century. “And 10,000 wouldn’t be a stretch,” says Kristofer Helgen, a mammalogist at the Smithsonian’s National Museum of Natural History, who has discovered roughly 100 new species.

Why now? New roads and rapid deforestation are opening up habitats once too remote to explore. Researchers sometimes discover new species just as hunting, farming and other pressures are pushing them to extinction. In addition, helicopters, satellite mapping, submersibles, deep-sea cameras and other modern tools help scientists methodically search understudied areas—including spots where wars or political barriers once kept them out.

A heightened sense of urgency about the threat of extinction has also encouraged international cooperation, sometimes on a global scale. For instance, the ten-year-long, 80-plus-nation Census of Marine Life will have discovered thousands of previously undescribed species—from a yeti crab to a giant spiny lobster—by the time it wraps up later this year.

Most future discoveries, says ornithologist Bruce Beehler of Conservation International, will probably come from remote areas with lots of variation in habitat—for instance, where a mountain range meets a river basin. In such terrain populations of organisms tend to get separated from one another and develop adaptations to survive in their new territory. Beehler says to expect discoveries from the eastern slope of the Andes in South America, the Congo basin in West Africa and the eastern Himalayas in Asia. On a 2005 helicopter expedition in New Guinea, he and Helgen discovered an entire “lost world” of new species deep in the Foja Mountains; after two return visits, the team has cataloged more than 70 new species, including a type of wallaby and a gecko. They now have their eyes on another mountainous area of western New Guinea they call “the Bird’s Neck.” They just need to figure out how to get there.

But new species also turn up in less exotic places—a slender salamander 30 miles from Los Angeles, or a new genus of tree that grows up to 130 feet tall two hours from Sydney, Australia. And Helgen notes that two out of three new mammal species are discovered in museum collection cabinets.

That’s partly because genetic analysis is revealing “cryptic species,” creatures that look alike to us but not to each other. For instance, scientists now believe that giraffes, currently classified as a single species, really belong to six or more species, some of which might not have bred together in the wild for more than a million years. Likewise, researchers recently took a closer look at a bat that ranges across much of South America and found genetic evidence suggesting that some identical-looking bats are different species. Such genetic differences can open the eyes of field biologists to previously unsuspected traits. “Maybe it’s scent, a sound, a pheromone, something that doesn’t get preserved in a museum,” says Elizabeth Clare of the University of Guelph in Ontario, a co-author of the bat study.

Why should we care? If you’ve seen one look-alike bat, or rat, or parasitic wasp, haven’t you seen them all? In fact, our own lives sometimes depend on recognizing the subtle differences. For instance, South American night monkeys of the genus Aotus used to be regarded as a single species. Then a primatologist found that they really belong to nine separate species that differ in their susceptibility to malaria. That mattered because scientists relied on Aotus as a laboratory animal for malaria studies—and did not realize they could be getting bogus results, and putting human lives in peril, by inadvertently testing malaria treatments on a species that might not be vulnerable to the disease in the first place.

But what really drives scientists to the far ends of the earth in search of new species is something far less pragmatic. Visiting New Caledonia as a young man, the evolutionist and ant taxonomist E. O. Wilson realized that “not just the ants but everything I saw, every species of plant and animal, was new to me.” Years later, the memory made him confess: “I am a neophile, an inordinate lover of the new, of diversity for its own sake.” His greatest desire was to live in a place “teeming with new life forms,” wrote Wilson, now 81. All he wanted was “not years but centuries of time” to take its measure.

Richard Conniff’s The Species Seekers will be out this fall.