Scientists Found Microplastics Deep Inside a Cave Closed to the Public for Decades

Scientists found high levels of plastic pollution in a Missouri cave system that has been closed to the public for 30 years, a discovery that suggests many subterranean habitats are contaminated by human pollution even in the absence of humans, according to two new studies.

An expedition team discovered anthropogenic (or human-sourced) microplastics, which are small particles of plastic, at every site they sampled in Cliff Cave, a system of labyrinthine karst passageways near Saint Louis that has been off-limits to the public since a fatal flash flood in 1993. The results reveal that microplastics, and other forms of anthropogenic pollution, may commonly infiltrate underground caves, posing potential risks to the fragile ecosystems that live in them and contaminating human water resources, such as aquifers.

Humans produce hundreds of millions of tons of plastic every year, a demand that generates an escalating flow of plastic waste. This runoff sometimes enters natural environments in large chunks, but it mostly breaks down into tiny particles that can spread far and wide; microplastics have been found in the deepest reaches of the ocean, in the air we breathe, and even in our blood.

Now, scientists led by Elizabeth Hasenmueller, associate director of the Water Access, Technology, Environment and Resources (WATER) Institute at Saint Louis University, report that microplastics, as well as cellulose-based pollutants, are present in the water and sediments of Cliff Cave, even though access to the cave is strictly limited for safety and conservation reasons. 

The results suggest that cave sediments “represent a potential source of ‘legacy’ pollution to the water resources and fragile habitats found in these globally distributed landscapes,” according to a new study published in Science of the Total Environment.

“Studies on microplastic debris initially focused on the ocean because of the highly visible problem of large plastic pollution in this environment. Recently, more research efforts have gone towards examining rivers, lakes, and other surface freshwater systems. However, we don’t know much about if and how these particles get into subsurface environments,” Hasenmueller told Motherboard. “Microplastic debris could be entering our groundwaters, a common drinking water resource, or caves, where fragile ecosystems exist. During the last few years, my research team has been focused on understanding microplastic prevalence and transport in these subsurface environments to assess their threat to water resources and subterranean ecosystems.”

Hasenmueller and her colleagues obtained special permits to explore Cliff Cave and extract samples of its water and sediments during day trips that took place in May 2019 and April 2022. As they pushed deeper into the cave, the team took samples every 25 meters (82 feet) until they reached a distance of about 180 meters (590 feet) from the entrance, beyond which access is further cut off by authorities due to flood hazards. 

“We found anthropogenic microparticles in all samples that were mainly fibers (91%) and clear (59%),” the team said. “Quantities in sediment were ~100 times those in water. These findings indicate that sediment sequesters anthropogenic microparticle pollution in the cave. Microplastic concentrations were similar among all sediment samples, but only one water sample at the main entrance contained microplastics.”

The researchers also learned that the anthropogenic microparticles were especially concentrated near the mouth of the cave, which suggests that airborne particles might settle at the entrance, contributing to the high concentrations there. 

“We expected that we would probably find microplastic debris in the cave given how common it is at the land surface, but we were surprised by how much we found in sediment samples from the cave compared to water samples: the sediment contained about 100 times more microplastic particles than the water!” Hasenmueller said. “That said, concentrations of the microplastics increased in the water when the cave experienced flooding after rainfall events. These floods transport the particles to the cave and we suspect that, as the flood water recedes, that material remains in the cave sediment, potentially for decades or longer.”

These occasional deluges increase the diversity of anthropogenic microparticles in the cave system, according to a separate study, co-authored by Hasenmueller, that was published last month in Water Research.

Altogether, the findings offer a rare look inside a cave that is largely off-limits to humans, but which is nevertheless contaminated with our plastic detritus. The effects of anthropogenic microparticles on these otherworldly habitats is still largely unknown, though Hasenmueller and her colleagues hope to better understand that question in the future.

“We are now working on determining the sources of microplastics to caves and groundwater by studying the land uses above these systems. We are also determining how long these materials may stay in the caves,” Hasenmueller said. “On the surface, microplastics can break down due to UV light exposure from sunlight, but this light source is not present belowground. So, we’re trying to determine if microplastics may last longer in subterranean ecosystems compared to surface habitats.”

Update: This article was updated with comments from team lead Elizabeth Hasenmueller.