Located deep within the Arctic Ocean, between Iceland and Norway, is a field of hydrothermal vents known as Loki’s Castle. A team of scientists recently extracted sediments from this environment, which is low in oxygen and high in pressure, hoping to learn more about the microbes that thrive there. What they found came as a shock: Multiple strains of Chlamydiae, some of them previously unknown to researchers.

Chlamydiae are a diverse group of bacteria, rendered notorious by Chlamydia trachomatis, the sexually transmitted infection that causes a range of uncomfortable symptoms in humans. All known members of Chlamydiae depend on other organisms—from animals to amoebas—for survival, living inside their hosts. The bacteria discovered on the Arctic floor were “abundant, diverse and active,” according to a new study in Current Biology—and yet there were no obvious signs of host organisms to be found in the ocean floor sediment.

“Finding Chlamydiae in this environment was completely unexpected, and of course begged the question: What on earth were they doing there?” says first study author Jennah Dharamshi, a cellular biologist at the Uppsala University in Sweden.

The newly discovered Chlamydiae were cored out from several feet beneath the Arctic sea floor, which sits some two miles beneath the surface of the water. Researchers analyzed DNA from 68 samples, and found that 51 of those samples contained Chlamydiae. One group, according to Nicoletta Lanes of Live Science, share a common ancestor with the bacteria that cause disease in humans and other animals.

Researchers were not able to grow the microbes in a lab—mimicking the extreme environmental conditions of the ocean floor is no easy feat—so it was difficult to study precisely how the deep sea Chlamydiae survive. In the absence of a host organism, the team suspects they might “require compounds from other microbes living in the marine sediments,” explains study co-author Thijs Ettema, a microbiologist at Wageningen University & Research in the Netherlands.

But given their relationship to pathogenic Chlamydiae, the newly discovered bacteria can help researchers understand how the lineage evolved its disease-causing qualities. The new study also challenges scientists’ conception of where and how Chlamydiae are able to survive; some of the new groups were “exceptionally abundant” in the ocean sediments, suggesting that they have an important impact on deep sea ecosystems.

“Chlamydiae have likely been missed in many prior surveys of microbial diversity”, says co-author Daniel Tamarit, a biologist at Uppsala University. “This group of bacteria could be playing a much larger role in marine ecology than we previously thought.”