Our planet is in the middle of a mass extinction — the sixth time in Earth's history that animal and plant species are disappearing in enormous numbers.

Amphibians, particularly frogs, are among the hardest hit by this extinction crisis, as are insects and reptiles.

At least 2,000 species of amphibian are in danger of extinction, according to a recent study. A report from the United Nations confirmed that 40% of amphibian species are threatened.

Here are photos of 15 endangered frogs, geckos, and snakes that might soon disappear.

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Human activity has killed off 680 mammal, bird, reptile, amphibian, and fish species since the 1500s. As much as half of the total number of animal individuals that once shared the planet with us are already gone.

That death toll is likely to rise dramatically over the next decades.

A recent report from the United Nations found that between 500,000 and 1 million plant and animals species face imminent extinction. At least 10% of insect species and more than 33% of all marine mammals and reef-forming coral are threatened, it found.

Read More: 17 signs we're in the middle of a 6th mass extinction

But one group is expected to suffer most of all: Amphibians. An estimated 40% of amphibian species face extinction, according to the UN report. A study published in the journal Current Biology estimated that at least 2,000 amphibian species are in danger of extinction.

This group includes frogs, toads, salamanders, and newts.

In the past 50 years, more than 500 amphibian species have experienced population declines worldwide, and 90 of them have gone extinct, due to a deadly fungal disease called chytridiomycosis (or chytrid fungus), which corrodes frog flesh.

According to a recent study in the journal Science, chytridiomycosis has wreaked havoc on frog, toad, and salamander species around the world. Amphibian deaths associated with chytrid fungus represent the greatest recorded loss of biodiversity attributable to any one disease, the authors found. Humans enabled the disease to spread further than it otherwise could have in large part because of the global wildlife trade.

Dutch photographer Matthijs Kuijpers has made it a personal mission to capture images of many colorful and bizarre amphibians before they disappear. His work is published in his new book, called "Cold Instinct" — take a look.