After a massive iceberg broke away from Antarctica, it revealed a long-hidden world of creatures on the seafloor

The researchers aboard the Polarstern research vessel found themselves in the right place at the right time last month. They were sailing near Antarctica's northern rim when a giant iceberg broke off the continent.

The ship was some 3,000 miles from the southernmost tip of South America, not far from the Brunt Ice Shelf, which is replete with giant, growing cracks. On February 26, one of those cracks tore through the shelf, and an iceberg of more than 490 square miles (1,270 square kilometers) splintered off. This known as a calving event.

The iceberg, named A74, is about the size of Los Angeles, and more than 20 times the size of Manhattan. As it moved away from Antarctica, it revealed a part of the sea floor that hadn't seen sunlight in 50 years.

The Polarstern crew waited for strong winds in the area to abate, then entered the gap between the A74 and the ice shelf on March 13. The scientists' goal: investigate the portion of the Weddell Sea bed that had been covered by up to 1,000 feet of thick ice for decades.

The Polarstern crew deployed a camera instrument called the Ocean Floor Observation and Bathymetry System, or OFOBS.

They towed it behind the the ship on a long cable, submerging the OFOBS up to half a mile under the surface.

The OFOBS recorded five hours of footage and took thousands of photos.

The crew also deployed buoys that could measure the temperature and salt content of the water in the newly created gap. Data from the buoys could tell scientists how quickly that part of the Antarctic is warming.

Once OFOBS reached the sea floor, it spotted various creatures living among stones that had tumbled into the water from the ice shelf above.

Most of the creatures the cameras spotted were sessile animals: organisms like anemones that attach themselves to rocks or the ocean floor and don't move.

Most of the species were filter feeders, like sea sponges.

These immobile creatures eat microscopic algae and other tiny organic particles in the water that float near their stony abodes.

According to Autun Purser, a member of the OFOBS team, the presence of these filter feeders wasn't a surprise. But some of the findings shocked his team.

"I was expecting fewer, larger filter feeding animals (sponges mainly)," Purser, an oceanic researcher with the Alfred Wegener Institute in Germany, told Insider via email from aboard the Polarstern.

That expectation was based, in part, on findings from a group of British researchers that also drilled more than half a mile into the ice shelf last month, 162 miles from the area Purser's team was exploring. That team found sponges living on stones under the ice.

Purser's team was startled to see many creatures swimming around, rather than staying stationary.

"I was not expecting to see octopus and fish, or many mobile animals, and they were actually there," he said.

The OFOBS spotted sea cucumbers, brittle sea stars, mollusks, worms, at least five fish species, and two types of octopus.

Using a grabbing device aboard the OFOBS, the scientists collected specimens of some of the creatures, as well as silt from sea floor.

The OFOBS also glimpsed a cute type of sea cucumber called a sea pig.

These translucent, water-filled creatures have tube-like legs — sometime on their heads — that help them scuttle about in the deepest, darkest parts of every ocean on the planet. 

It's a bit of a mystery what, precisely, sustained this diverse, underwater ecosystem in the absence of sunlight. Most organic food stuffs and algae hang out in parts of the ocean where they have access to the light they need to survive.

By collecting more seafloor samples and visiting this area of the Weddell Sea again in the future, the Polarstern researchers hope to answer that question.

Purser said the team's observations show that marine ecosystems can be quite diverse and abundant, even if there's only a moderate amount of food available.

"Possibly everything happens over a longer timeframe, animals more slowly growing, etc.," he added. "But to determine if this is the case, repeat observations of the under-ice community, whilst still under ice, would be needed."

The team hopes to one day use autonomous underwater robots to investigate parts of the ocean that were formerly trapped under the ice.

This isn't the first time Antarctica has lost a giant iceberg, and it won't be the last.

"It is entirely natural for sections to calve away from ice shelves. As ice flows off the land, the ice shelf grows and eventually reaches a size which is unstable," Adrian Luckman, a glaciologist at Swansea University in Wales, previously told Insider. "Some calving events are small and go unnoticed, but every few years a large one such as this happens."

Researchers aboard the Polarstern said icebergs the size of A74 slough off every decade or so.

In 2017, an iceberg the size of Delaware broke off the continent's Larsen C Ice Shelf.

Luckman thinks another iceberg will break off the Brunt Ice Shelf in the coming weeks or months.