 -- A 19th century shipwreck buried underground in Boston is being excavated today after it was unearthed by chance late last week.

Construction workers stumbled upon the 50-foot-long boat while beginning work on a new building in the seaport area of the city, according to Charley Leatherbee, vice president of development at Skanska USA Commercial Development.

"During normal excavation operations, our crews discovered something unusual that we believe may be a piece of a boat or a boat hull," Leatherbee told ABC News today in a statement. "We immediately halted our excavation operations in the area of the site, and alerted the city archaeologist and Massachusetts Historical Commission."

Boston's city archaeologist, Joseph Bagley, told ABC News today the find was "incredibly rare," explaining that "there's only been one other terrestrial shipwreck that's been found in Boston, and that one was only very small."

The ship likely sunk sometime in the mid- to late-1800s, when what is now land used to be a mud flat that would be submerged during a high tide, Bagley said.

"It appears that the ship must have been full of barrels of lime," he said. "It looks like the barrels were from Maine, and they were probably being shipped here for the development industry to mix with mortar."

But Bagley pointed out that lime in large quantities mixed with water can cause a chemical reaction. He believes that the barrels likely caught on fire, causing the ship to burn down, he added.

"We've been working all day under the assumption that this will be are only opportunity to look at this stuff," Bagley said. "Skanksa actually stopped construction, even though they didn't have to legally, just to allow us to review the ship. We're really grateful."

What will happen to the shipwreck after today remains "up in the air" and up to Skanska, Bagley said.