It’s easy to assume, as many people do, that our planet is well explored. In the last few centuries, humans have summited Earth’s highest peaks, dived its deepest ocean trenches, and trekked to the North and South poles, documenting the diversity of life along the way — the many birds, butterflies, fish, and other creatures with which we share our big planet.

Life on Earth is now largely known.

Except it isn’t.

The more that scientists study the planet’s biodiversity, the more they realize how little of it we know. They estimate that for every species we’ve discovered, there are likely at least another nine or so that remain undiscovered or unidentified, meaning around 90 percent of life on Earth is unknown.

This doesn’t include the big stuff — the black bears and belugas and bald eagles, all of which have scientific names and descriptions published in academic journals. The unknown is made up of small organisms, such as insects, mites, and crustaceans. These species are the nuts and bolts of ecosystems: They produce soil, pollinate crops, and feed almost everything. And most of them have yet to be identified.

In just one fly family known as Cecidomyiidae, for example, scientists estimate there could be as many as 1.8 million species globally, and yet fewer than 7,000 have been described. This is especially remarkable given that the total number of described species across the entire animal kingdom is somewhere around 2 million.

Biologists describe animals like this as dark taxa, a term that refers to groups of organisms in which the bulk of species are undescribed or undiscovered. Some taxonomists have also called them biology’s dark matter.

“Most people think that life on Earth is described, and we have a good idea of how ecosystems are functioning,” said Emily Hartop, a fly researcher and taxonomist at the Norwegian University of Science and Technology, who studies dark taxa. “The reality is that for most species on Earth, we don’t know what they are, we don’t know where they are, we don’t know what they’re doing. They are unknown.”

Scientists who study dark taxa argue that lifting the shadow on these organisms is essential to our own survival. If we don’t know what constitutes our ecosystems, we risk killing off the key players that make them function — or failing to detect a potential threat, such as a disease-carrying insect that could set off the next global pandemic.

“The little things run the planet,” said Rudolf Meier, a researcher at Berlin’s Museum of Natural History and Humboldt University of Berlin who also studies dark taxa.

Hartop and some other researchers have dedicated their careers to exposing dark taxa — to making........

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