The unexpected finding came when sonar equipment on board a search vessel scouring the Indian Ocean for the missing jetliner detected a cluster of objects nearly 2.5 miles below the surface, according to a statement from the Australian Transport Safety Bureau, which is heading up the hunt.
Although officials suspected the objects were probably not from the Boeing 777, which vanished on March 8, 2014, they decided to take a closer look just in case. A second ship sent down an autonomous underwater vehicle - essentially, an unmanned sub - which revealed a large number of small objects and several larger items, the biggest 20 feet long.
The debris field appeared to be man-made but wasn't typical of an aircraft. Still, crews sent down a camera to be sure.
Analysis of the photos revealed this week that the debris came from a previously uncharted shipwreck. Marine archaeologists are now examining the photos, which include an image of an anchor and what appear to be lumps of coal, to see whether they can identify the ship. It was not immediately clear when the sonar first spotted the wreckage.
"It's a fascinating find, but it's not what we're looking for," Peter Foley, the ATSB's Director of the Operational Search for Flight 370, said in a statement. "We're not pausing in the search for MH370, in fact the vessels have already moved on to continue the mission."
Michael McCarthy, a senior maritime archaeologist at the West Australian Maritime Museum, said the wreck was of a cargo ship built in the mid-to-late 19th century, and could be one of hundreds lost during voyages across the Indian Ocean.
"We've got quite a lot of stories about ships that sank in the Indian Ocean mid-voyage and you would be struggling to tell which is which unless you had a complete catalogue of all the ones lost," he said.
Experts had predicted Flight 370 search crews would probably stumble across the wreckage of some ships, given that they once sank regularly due to old age or bad weather, McCarthy said. But it would be difficult to identify this particular wreck without getting a closer look and knowing which ports it was traveling between, he said.
"Being a fairly common type of cargo ship from the 19th century with no obvious cargo remains there, I doubt that anyone would pay the enormous cost of going down to look at it," McCarthy said.
Last month, officials announced that they would expand the search area for Flight 370 by another 23,000 square miles if the plane is not found by the end of May. Crews have now covered 75 percent of the original search area and are working in the southern portion of the expanded search zone to take advantage of the last dregs of decent weather before winter sets in.
One of the four search vessels, which has the autonomous underwater vehicle on board, has withdrawn from the hunt because the worsening weather has made it too difficult for crews to launch the sub.
Foley said all is not lost with the find.
"Obviously, we're disappointed that it wasn't the aircraft, but we were always realistic about the likelihood. And this event has really demonstrated that the systems, people and the equipment involved in the search are working well," Foley said. "It's shown that if there's a debris field in the search area, we'll find it."