There are holes on the ocean floor. Scientists don’t know why.

Deep in the waters along a volcanic ridge on the floor of the Atlantic Ocean, marine explorers using a remotely operated vehicle to explore largely unexplored areas discovered a pattern of holes in the sand.

During the dive, north of the Azores, near mainland Portugal, on July 23, they saw about a dozen groups of sinkholes resembling a trail of lines on the ocean floor at a depth of 1.6 miles.

Then about a week later, on Thursday, there were four more sightings on the Azores Plateau, which is an underwater terrain where three tectonic plates meet. These holes were about a mile deep and about 300 miles from the site of the expedition’s original discovery.

The question that scientists pose to themselves and to society in publications of Twitter and Facebook, is: What creates these scars on the ocean floor?

“The origin of the holes has baffled scientists,” said a Twitter post from the National Oceanic and Atmospheric Administration’s Ocean Research Project. “The holes look man-made, but the small piles of sediment around them suggest they were dug by…something.”

Nearly two decades ago, just about 27 miles from the site of the current expedition’s original observation, scientists noticed similar holes during a survey, said Emily Krum, a NOAA spokeswoman.

But the passage of time has not provided clear answers, said Michael Vecchione, a NOAA deep-sea biologist who is involved in this project and also participated in part of this latest expedition.

“There’s something important going on there, and we don’t know what it is,” Dr. Vecchione said. “It highlights the fact that there are still mysteries.”

The sinkholes are just one of the questions scientists on an ambitious ocean expedition are exploring as they explore the Mid-Atlantic Ridge, which is part of a massive deep-ocean ridge that stretches more than 10,000 miles beneath the Atlantic Ocean.

NOAA experts are looking for answers during three expeditions they’re calling “Voyage to the Ridge 2022,” which began in May and will end in September, on trips that take them from the waters off Newport, Rhode Island, to the Azores and back to Puerto Rico in the Caribbean.

Researchers want to know what lives in the continuous range of underwater volcanoes and what happens when the geological processes that create life-sustaining heat are stopped.

They pay close attention to deep-sea coral and sponge communities, which are “some of the most valuable marine ecosystems on Earth,” said Derek Sowers, expedition coordinator aboard the NOAA ship Okeanos Explorer.

Dr Sowers said expeditions such as the Voyage of the Ridge projects were “fundamental” to establishing an understanding of the planet’s biodiversity and “the new compounds produced by all these life forms”.

And they want to know more about the areas where seawater is heated by magma, with deep-sea life drawing energy from that source and chemicals instead of the sun like most life on Earth.

“This has expanded our understanding of the conditions under which life might arise on other planets,” Dr Sowers said.

After the agency took to social media in an effort to engage the public, dozens of comments emerged, some delving into speculation. Are the holes artificial? Could they be a sign from aliens? Are there tracks left by a submarine? Could they be the breath of “a deep-sea creature that burrows under the sand?”

The latter assumption is not necessarily so far-fetched, Dr. Vecchione said. In a paper about the holes spotted in 2004, Mr. Vecchione and his co-author, Od Axel Bergstad, a former researcher at the Norwegian Institute of Marine Research, proposed two main hypotheses for why the holes exist. Both involved marine life walking or swimming above the sediment and drilling holes downward, or the reverse scenario, burrowing into the sediment and drilling holes upward.

The holes seen on Thursday appeared to have been pushed from below, Dr Vecchione said.

The remote-controlled vehicle’s suction device collected sediment samples to check for an organism in the holes, Dr. Sowers said.

Dr Vecchione said that while he was pleased to come across the holes on the ocean floor again, he was “a bit disappointed” that scientists still had no explanation.

“It reinforces the idea that there is a mystery that we will understand someday,” he said. “But we haven’t figured it out yet.”

One final live dive remains to be done on the second expedition in the series, NOAA said. The third expedition begins on August 7.

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