Thousands of mystery holes have been discovered on the seafloor after deep-sea surveys.

The Earth oceans are anything but explored. According to reports, humans have managed to explore about 5% of the ocean floor. The remaining 95% of the ocean still remains a mystery. That’s why finding out that there are thousands of strange holes beneath the surface does not come as a great surprise.

A few miles off the coast of Big Sur, California, an underwater survey has found thousands of circular holes scattered in the soft sediment of the seabed. Scientists still don’t know what they are nor what may have caused them.

So, what are the mysterious holes that decorate the seafloor? Are they natural or artificial in nature?

As explained by experts from the Monterey Bay Aquarium Research Institute (MBARI), experts discovered the presence of two distinct sizes.

The larger holes are known as pockmarks and average around average 175 meters (almost 600 feet) across and five meters (16 feet) deep. These holes appear to be nearly circular and are found to be fairly evenly spaced.

Some of these mysterious pockmarks were found by experts in 1999 during a seafloor survey using ship-mounted sonar.

Since then, additional surveys of the seabed have revealed the presence of more than 5,200 pockmarks spread out over 1,300 square kilometers (500 square miles), which makes this area the largest known pockmark field anywhere in North America.

A recent survey of the seabed with the help of sonar mounted on autonomous underwater vehicles revealed the existence of thousands of smaller pits which experts have dubbed micro-depressions. These average around 11 meters (36 ft) across and are around one meter deep.

Although seafloor pockmarks have been discovered in other parts of the world, the ones discovered off the coast of California are unique.

The pockmarks found elsewhere are associated with the release of methane gas or other materials from the seafloor.

However, the survey of the area by MBARI experts has revealed no evidence of methane or other similar gasses in the seafloor.

Furthermore, sonar data of the seafloor sediments indicates that the pockmarks near California have remained inactive for around 50,000 years.

The smaller micro-depressions, however, are thought to have formed in relatively young sediment. Furthermore, most of the micro-depressions which have been studied by experts are home to kelp holdfasts, bones, trash, or fishing gear. However, despite the fact that the depressions were home to countless artifacts, the sediment around the holes was found to be empty.

The survey has also revealed that most of the micro-depressions feature “tails” of sediment that most likely originated within the depression. Strangely, many of these tails are oriented in the same directions adding fuel to the mystery.

Scientists theorize that the micro-depressions are young features that were excavated by seafloor currents not long ago.

“The pockmarks and micro-depressions in this area are both holes in the seafloor that occur in softer sediments, but they are morphologically distinct. The cause and persistence of the pockmarks still remain a mystery, but we find no evidence they were created from gas or fluid in the seafloor in the recent past,” Eve Lundsten a researcher with MBARI revealed in a statement.

“The micro-depressions are recently formed erosional features; they are not ‘incipient pockmarks.’ Overall, a lot more work needs to be done to understand how all these features were formed, and this work is in progress,” Lundsten added.

The area is being considered as the position for an offshore wind farm, the reason why experts surveyed the site recently. The exact origin of the mystery holes on the seafloor remains a mystery.