Trapped deep underneath the surface and left to evolve in isolation for thousands of years, cave animals are some of nature’s most bizarre and fascinating creatures. Scientists call them “troglobites,” and some species are so rare that they consist of a handful of individuals in a single cave.

Cave life is evolution at its most extreme, but troglobites are more common than you might think. Any time people explore new caverns, there’s potential for finding new species. Here’s our list of 10 incredible cave animals that have evolved to live in the darkness.

Olm

This eyeless, white, dragon-like amphibian is called an olm and lives in the karst caves (made from soluble rocks like limestone, gypsum, and dolomite) of Slovenia and Croatia.

Describing it as a dragon isn’t that far from the truth. When first discovered in the 18th century, many people believed the creatures were baby dragons, a belief reinforced by their dark, aquatic cave habitat.

The olm is likely the first troglobite discovered, and to date it is also the largest. Some olms measure as much as one foot in length.

Water pollution highly threatens olms. The IUCN has listed them as a vulnerable species due to the fragmentation and degradation of their habitat.

Cave Pseudoscorpion

These cave animals look like the hybrid offspring of a spider and a scorpion, but pseudoscorpions belong to an arachnid order all to themselves. Despite looking like tailless scorpions, they are most closely related to camel spiders and have no stingers. There are more than 3,500 species of pseudoscorpion worldwide, large numbers of which call caverns home. Some of these species are limited to single caves.

Cave pseudoscorpions differ from their aboveground relatives in that they only have a single pair of eyes or no eyes at all. Terrestrial pseudoscorpions have two sets of eyes.

In 2010, scientists discovered a new species of pseudoscorpion with venom-filled claws living in the deep granite caves of Yosemite National Park.

Kaua’i Cave Wolf Spider

Scientists discovered the Kaua’i Cave Wolf Spider in 1971, in a few lava tubes on the Hawaiian island of Kaua’i. This eight-legged predator is called the blind wolf spider by locals and is one of the rarest creatures in the world. In fact, researchers have never documented more than 30 spiders at a time.

The wolf spider’s closest living surface-dwelling relative has large eyes, like most types of wolf spiders. Still, the Kaua’i wolf spider has wholly lost its eyes because of the realm where it lives in isolation and darkness.

Its favorite prey is another cave-dwelling creature, the Kaua’i cave amphipod, which numbered at most 80 in surveys. This endangered spider is particularly threatened by humans using their cave habitat as a place for parties. Nicotine in cigarettes is a potent insecticide, and the toxic fumes harm the spiders and other cave inhabitants. Likewise, the trash left behind attracts nonnative insects like cockroaches and ants that then attract non-native predators.

Cave Harvestman

Harvestmen species occur in caves worldwide. Much of the research on harvestmen happens in Brazil, the home of over 1,000 described harvestmen species. In the United States, landowners have unsuccessfully brought court cases in an attempt to overturn endangered species protections for cave harvestmen.

Harvestmen are another cave species that looks like something nearly unrelated. In this case, a cave harvestman looks like a spider but is a separate arachnid order, called Opiliones. Other members of this order are the “daddy-long-legs” found on the surface.

These animals are well-adapted to cave life and are some of the most commonly found types of troglobite. Troglobitic harvestman species lack the unneeded eyes and the camouflaging coloration that protects surface Opiliones.

Tumbling Creek Cave Snail

This aquatic cave snail lives on the undersides of rocks inside a cave in the Tumbling Creek area of southern Missouri. They live in areas with large deposits of bat guano. Scientists believe that they may rely on guano biofilm runoff as a source of nutrition.

Although more than 15,000 individuals existed at the time of their discovery, water pollution severely depleted their numbers with some U.S. Fish and Wildlife Service surveys failing to find any. A landowner named Tom Aley has worked hard to help protect the Tumbling Creek cave snail and the other endangered species that call the area home.

Devil’s Hole Pupfish

This fish is so rare that it is found only in a single aquifer-fed pool within a limestone cavern in Death Valley National Park. This environment is unusual for fish, as it has 93-degree water with deficient oxygen levels. These fish only manage to live for about a year, but they spend that time frolicking like puppies, hence their name.

Despite relying on a shallow limestone shelf of only 2 meters (6.6 feet) by 4 meters (13 feet) for spawning, it has survived as a species for at least 22,000 years. It helps being small, with adults measuring only 35 millimeters, about the size of a goldfish.

Unfortunately, for unknown reasons, the already limited population declined significantly, beginning in the late 1990s. The fish have low fecundity, unfortunately. Surveys in the fall of 2018 and the spring of 2019 brought the good news that the conservation actions taken are reversing the decline.

Cave Crayfish

While cave crayfish occur worldwide, but the southeastern United States is thought to have the most crayfish species, particularly Alabama and Florida.

Troglobites have adapted to cave life, which often offers a limited food supply. As a result, they typically have slow, energy-efficient metabolisms. Scientists used the southern cave crayfish (Orconectes australis) as the textbook example of a long-lived species, claiming they lived 176 years because of slow metabolism. However, repeat studies failed to show that this extraordinary lifespan is typical. Cave crayfish do show other adaptations to cave life, such as a lack of pigmentation, longer antennae, and blindness.

Cave Beetle

Despite the discovery of the olm in 1689, scientists didn’t believe caves were a suitable habitat for plants or animals until a lamplighter in the same caves in Postojna, Slovenia, found a cave beetle, Leptodirus hochenwartii in 1831. Like the cave crayfish, many species of cave beetles exist in the southern United States, with over 200 species in one genus.

Cave beetles feed on the fungi and bacteria that enter the cave via animal droppings. Cave beetles show the same adaptations as other troglobitic creatures: longer antennae and spider-like appendages, lower food needs, lack of functional eyes and flight wings, and no pigmentation.

Blind Cavefish

A surveyor first discovered blind cavefish in 1936 in the karst caves of Sierra de El Abra in Mexico. Genetic studies show that surface-dwelling populations of this fish invaded three separate caves and rapidly evolved into the eyeless, unpigmented cave lineages.

In Mexican cavefish, fish that live in pools with no surface light cannot see and are eyeless. Those that have some access to light through a surface river that goes underground have slightly diminished vision.

Blind cavefish use sonic clicks to communicate with others in their school. Researchers have identified up to six different kinds of clicks, some more aggressive than others when competing for food. There appear to be distinct “accents” for each cave-dwelling population.

Texas Blind Salamander

Found only in the underground water systems of Edwards Plateau in Texas, this troglobite salamander is another underworld amphibian that could easily be mistaken for a baby dragon. Adults are 3.25 to 5.375 inches in length, have red gills on the back of their heads, and are otherwise colorless. Like most troglobites, they have lost their sense of sight, an adaptation to their dark habitat. When hunting for food, they move their head from side to side to sense a change in water pressure to locate prey.

As an aquatic species with a very restricted range, they are under threat from water pollution.