You don’t have to look far to see we’ve descended into the Anthropocene, a period dominated by human impacts on this planet—our moving of mountains and waterways, our corruption of the climate, the traces of nuclear material in the geological record. Add to all that microplastic pollution, an increasingly pervasive threat that’s swirling in the ocean and finding its way to distant corners of the Arctic.

Today in Science Advances, researchers at the Scripps Institution of Oceanography report how the Earth itself is tainted with microplastic particles. By taking a core sample of sediments just off the coast of Southern California, the scientists could observe in its layers how plastic concentrations have changed year after year. And exponentially so: Since the 1940s, when plastic production began to take off, microplastic deposition rates have doubled every 15 years. This correlates with both figures on plastic manufacturing and coastal population growth in California, and brings us to a troubling conclusion: As seaside cities continue to boom, so does the amount of microplastic flowing into the sea, tainting whole ecosystems.

The researchers got their sediment samples from something called a box core, essentially a giant cookie cutter that slices down many years’ worth of layers in the seafloor. Back at the lab, they dried each layer and ran the material through filters to isolate the particles, which they counted visually under a microscope and tested chemically to determine the variety of plastic.

Interestingly, two-thirds of the particles the researchers found were fibers. These are coming largely from synthetic clothing like yoga pants, which slough off fibers in the wash. A wastewater treatment plant processes that water before pumping it out to sea, but isn’t equipped to remove all the microfibers. “There's just this steady onslaught of microfibers reaching the bottom of the ocean,” says Scripps oceanographer Jennifer Brandon, lead author on the new paper. “Microfibers for a tiny animal like plankton can act like a rope would for us—they can entangle them, they can get caught in their guts, they can kind of pinch their limbs.”

In addition, macroplastics like single-use bags float out to sea and bake in the sun, causing them to break into much smaller fragments that then swirl in the water column. Then it’s only a matter of time before ocean organisms ingest the particles—giant larvaceans, for instance, rely on a mucus net to catch tiny prey, a haul now sullied with microplastics. Once they discard their mucus nets, the apparatus sinks to the seafloor, dragging the plastics down as well. And that’s just one way microplastics can move up and down the water column and settle in the mud.

Then there’s the question of color, which is often overlooked in research on plastic pollution. Although microplastics come in a galaxy of hues, Brandon and her colleagues found that most of their particles were white. Many marine predators choose their prey based on color, so they could be mistaking white microplastic particles for, say, a transparent planktonic organism with eggs in its belly. “It's happening and we're not talking about it enough, that's for sure,” says Brandon.

The tricky bit about microplastic pollution is that little is known about how the pollutant might be affecting organisms, and in turn whole ecosystems. It’s virtually impossible to do a controlled study out in the sea. (Though researchers may soon use remote lakes in Canada to do experiments.) And when scientists are working in the lab, they’re exposing organisms like bacteria to unnaturally high concentrations of plastic to elicit a physiological response.

The concentration of microplastics off the coast of California, where these Scripps researchers did their work, might even be relatively low compared to other parts of the world, making it harder to observe their effects on organisms. “If they were doing the same thing in the Yellow Sea in China, right outside some of the big rivers like the Yangtze and Yellow River, the concentrations would probably be huge and cause adverse effects,” says University of Michigan eco-toxicologist Allen Burton, who studies microplastics.

What’s particularly troubling is that as coastal populations continue to grow in China and elsewhere, plastic will continue to pile up in the sea, be that by litter or tainted wastewater. At the same time, plastic production is skyrocketing—humans made 400 million tons in 2015, and that’s expected to double by 2025.

All that plastic doesn’t easily degrade—it is, after all, engineered to be tough. “They will still be in sediment cores for future civilizations to find them, because except for bacteria it doesn't look like most things can degrade them in any way,” says Burton. “They break down into smaller and smaller pieces but they're still inherently, chemically plastic. We'll find them like we find old artifacts.”

So given its omnipresence and its ability to persist in the environment, is microplastic a good marker for the Anthropocene, humanity’s era of extreme environmental meddling? “Why wouldn't it be a good marker,” says Burton, “if plastic started in the mid-’40s and we've got all the numbers for plastic production and we know how it's going up?”

Welcome to Planet Plastic. This is our shameful environmental legacy.

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