Fallout from humanity’s nuclear legacy is locked in glaciers around the world, and now scientists are worrying that it may seep into the environment as climate change causes Arctic ice to melt.

This apocalyptic scenario was explored by an international team of scientists who looked at the spread of radioactive contaminants in Arctic glaciers throughout Sweden, Iceland, Greenland, the Norwegian archipelago Svalbard, the European Alps, the Caucasus, British Columbia, and Antarctica. The researchers shared their results at the 2019 General Assembly of the European Geosciences Union (EGU) in Vienna last week.

Fallout radionuclides (FRNs) were detected at 17 locations at these sites. Radioactive material was found embedded within ice surface sediments called “cryoconite,” and at concentration levels ten times greater than the surrounding environment.“They are some of the highest levels you see in the environment outside nuclear exclusion zones," Caroline Clason, a lecturer in Physical Geography at the University of Plymouth who led the research project, told AFP on Wednesday.

The scientists blamed the advent of nuclear weapons testing during the 1950s and 1960s for much of the debris. The Chernobyl disaster in 1986 and the Fukushima meltdown in 2011 were also responsible for some fallout.

“If we take a sediment core you can see a clear spike where Chernobyl was, but you can also see quite a defined spike in around 1963 when there was a period of quite heavy weapons testing,” Clason said.

Weapons tests can fling radioactive detritus up to 50 miles in the air. Smaller, lighter materials will travel into the upper atmosphere, and may “circulate around the world for years, or even decades, until they gradually settle out or are brought back to the surface by precipitation,” according to the Environmental Protection Agency.

Fallout is comprised of radionuclides such as Americium-241, Cesium-137, Iodine-131, and Strontium-90. Depending on a material’s half-life, it could remain in the environment minutes to years before decaying. Their levels of radiation also vary.

Particles can return to the immediate area as acid rain that’s absorbed by plants and soil, wreaking havoc on ecosystems, human health, and communities. But radionuclides that travel far and wide can settle in concentrated levels on snow and ice—large amounts of radioactive material from Fukushima was found in 2011 on four glaciers in the Tibetan Plateau, for example.

Americium, which Clason called “particularly dangerous” for being more soluble in the environment and a stronger radiation emitter, and is produced by the decaying of plutonium, as well as caesium were found at some of the glacier sites, with Switzerland's Morteratsch glacier containing the highest caesium levels, according to ScienceAlert.

The team hopes that future research will investigate how fallout could disperse into the food chain from glaciers, calling it a potential “secondary source of environmental contamination many years after the nuclear event of their origin.”

A study published this week in Nature surveyed 19,000 of Earth’s glaciers and found their total melt amounts to a loss of 335 billion tons of ice each year, more than measurements of previous studies.