American scientists have discovered a mysterious dome-shaped specimen deep on the Alaskan seafloor, but nobody knows for sure what it is.

Ocean researchers using a remotely operated survey vehicle were amazed last week when they spotted the strange item on a rock about 3,300 meters (two miles) deep in the Gulf of Alaska.

It is over 10 centimeters (4 inches) in diameter and had a small tear near its base.

"As cameras zoomed in, scientists were stumped as to its identification, with initial thoughts ranging from a dead sponge attachment, to coral, to an egg casing," the National Oceanic and Atmospheric Administration said in a statement.

"Invoking almost fairytale-like imagery, the specimen has since been dubbed a 'golden orb' and even a 'golden egg,'" the agency added.

"Isn't the deep sea so delightfully strange?" wondered Sam Candio,  an NOAA Ocean Exploration coordinator.

As theories swirled on social media, including that of it being an alien egg, scientists extracted their August 30 discovery from the ocean floor to analyze it in a laboratory setting.

"While we were able to collect the 'golden orb' and bring it onto the ship, we still are not able to identify it beyond the fact that it is biological in origin," NOAA said.

It remains unclear if the golden dome is associated with a known species, a new species, or represents an unknown life stage of an existing one, according to Candio.

"While somewhat humbling to be stumped by this finding, it serves as a reminder of how little we know about our own planet and how much is left to learn and appreciate about our ocean," he added.

The dive was part of an expedition in the Gulf of Alaska to explore deepwater habitats.