The largest ecosystem on Earth – the deep sea – is one of the least studied even as climate change, plastic pollution and seabed mining threaten its biodiversity.

Those are the findings from a new review of studies on life in the deep sea, the area of the ocean below 650 feet (200 meters) that covers about two-thirds of the planet’s surface.

In reviewing all the papers published on the population genetics of deep ocean invertebrate life since 1970, researchers found that the number of studies – just 77 – is “minuscule in relation to the size of the deep sea.”

That lack of knowledge, they say, has made it difficult to draw broader conclusions about deep-sea biology, limiting the ability to craft effective management and conservation strategies despite what they call the “ever-increasing encroachment of human activity in the deep sea.”

“Unfortunately, as it is so rarely studied, we have no grasp of background diversity levels, let alone rates of species loss,” said lead author Michelle Taylor, a postdoctoral researcher at Oxford University, adding that it’s “shocking” how little is known about the largest habitat on Earth. “Even conservative estimates state that we have only looked at a tiny portion of the deep sea floor.”

Humans have explored less than 5 percent of the ocean, according to the National Oceanic and Atmospheric Administration, though other researchers note that more than 5 percent of the sea floor has been mapped, though at poor resolutions.

But the last review of knowledge of deep-sea life was 20 years ago, according to Taylor. The expansion of fishing, mining and other industries into that habitat meant it was time for an update.

Of the studies that have been done – the review excluded studies of hydrothermal vent species – the researchers found most have focused on larger animals, or what they call “more conspicuous megafauna.” Many of those were on species with commercial potential, such as deep-water shrimp, octopus, crabs and sea snails.

The studies also skewed toward relatively shallow depths, above 6,500 feet (2,000 meters), beyond which there is currently little fishing or mining. But the average ocean depth is around 12,000 feet (3,700 meters), and the mining of the seabed for valuable minerals is likely to begin within the decade. The lack of knowledge about the biodiversity of those depths could make it difficult to determine or forecast the impacts of mining.

Published studies also focused on certain regions, mainly those near industrialized countries. Eighteen percent of the 77 studies published on the population genetics of deep-sea invertebrates since 1970 focused on the North Atlantic and 48 percent on the Atlantic more broadly. The Pacific, twice as large, was examined in just 30 percent of the studies. Just 7 percent of the studies focused on the Indian Ocean, which is mainly surrounded by developing nations that depend on the ocean for protein and livelihoods. Fewer than 4 percent of studies focused on the ocean as a whole.

“Unlike the depth skew, this geographical bias almost certainly does not match the geographical intensity of human resource exploitation in these deep-sea regions,” the paper noted.

A few general patterns about deep-sea life could be inferred from the existing studies.

For one thing, the existing studies show that the formation of new species may happen at a similar rate in the deep sea as at other depths. And species’ ranges appear to extend for thousands of kilometers – but only between a few hundred specific meters in depth.

This limited “vertical connectivity” – the reasons for which are not entirely clear, according to the study – stood out as particularly interesting to Lance Morgan, president of the Marine Conservation Institute and chair of the Deep Sea Conservation Coalition.

“The more we learn about the deep sea, I think we recognize there’s a lot more we don’t know,” said Morgan, who was not involved with the study. “There’s a lot of diversity we don’t understand.”

It’s possible the consequences of that lack of understanding may never become fully clear if activities like mining or impacts from climate change erode deep-sea biological diversity before the gaps in knowledge are filled in.

“I don’t know if time is running out, but certainly things are changing much faster than we can appreciate,” said Morgan. “I think we all know we’re dealing with a very data-poor environment and as a result, we don’t have a lot of confidence that what we’re doing is at all sustainable in the deep sea.”

He noted that the deep sea may be already changing because of its ability to absorb heat and carbon dioxide impacts from climate change, “so we have that on top of the possibility of deep sea mining and things like that.”

Those human impacts also include plastic pollution. A study published earlier this month described waters below 7,000 feet (2,200 meters) as the “final sink” for microplastics, with concentrations of the pollutants on par with those of surface waters and nearly half of deep-sea invertebrates sampled having ingested plastics in amounts comparable to shallow-water species.

“Humans are encroaching into the deep sea ever more frequently,” said Taylor. “Fishing now commonly occurs down to 2,000 meters, deep sea extractions of gas have been occurring for a few decades, and deep sea mining is imminent.”

She said that makes the study particularly timely. “Is impacting one area of the deep sea impacting a breeding location, a source or a sink location, breaking a connectivity highway? Such information is required by industry and area managers but is lacking,” Taylor said.

The paper did find an uptick in studies on deep-sea life since the 1980s, with an average of one paper on deep-sea species published every year from 1991 to 1995 and five papers a year between 2011 and 2015. The peak, eight papers, came four years ago in 2012.

More studies gathering more data could spur further studies. As could targeting some of the research biases the authors noted in the paper. Now that those gaps and biases are known, Taylor said, “We hope that funding agencies, industries and governments can more easily target geographical, taxonomic and depth gaps with expeditions and research.”

But for now, the deep sea remains the least understood habitat on Earth. “We may know more about the surface of Mars and the moon than what’s going on in the deep sea,” said Morgan.

This version corrects an earlier version of the story in which it was reported that a review found that only 77 studies on deep-sea invertebrates have been published since 1977. In fact, the review was restricted to studies of the population genetics of deep-sea invertebrates.