The tablet went on display at the British Museum on Friday, and soon engineers will follow the ancient instructions to see whether the vessel could actually have sailed.
It's also the subject of a new book, "The Ark Before Noah," by Irving Finkel, the museum's assistant keeper of the Middle East and the man who translated the tablet.
Finkel got hold of it a few years ago, when a man brought in a damaged tablet his father had acquired in the Middle East after World War II. It was light brown, about the size of a mobile phone and covered in the jagged cuneiform script of the ancient Mesopotamians.
It turned out, Finkel said Friday, to be "one of the most important human documents ever discovered."
"It was really a heart-stopping moment — the discovery that the boat was to be a round boat," said Finkel, who sports a long gray beard, a ponytail and boundless enthusiasm for his subject. "That was a real surprise."
And yet, Finkel said, a round boat makes sense. Coracles were widely used as river taxis in ancient Iraq and are perfectly designed to bob along on raging floodwaters.
"It's a perfect thing," Finkel said. "It never sinks, it's light to carry."
Elizabeth Stone, an expert on the antiquities of ancient Mesopotamia at New York's Stony Brook University, said it made sense that ancient Mesopotamians would depict their mythological ark in that shape.
"People are going to envision the boat however people envision boats where they are," she said. "Coracles are not unusual things to have had in Mesopotamia."
The tablet records a Mesopotamian god's instructions for building a giant vessel — two-thirds the size of a soccer field in area — made of rope, reinforced with wooden ribs and coated in bitumen.
Finkel said that on paper (or stone) the boat-building orders appear sound, but he doesn't yet know whether it would have floated. A television documentary due to be broadcast later this year will follow attempts to build the ark according to the ancient manual.
The flood story recurs in later Mesopotamian writings including the "Epic of Gilgamesh." These versions lack the technical instructions — cut out, Finkel believes, because they got in the way of the storytelling.
"It would be like a Bond movie where instead of having this great sexy red car that comes on, somebody starts to tell you about how many horsepower it's got and the pressure of the tires and the capacity of the boot (trunk)," he said. "No one cares about that. They want the car chase."
Finkel is aware his discovery may cause consternation among believers in the Biblical story. When 19th-century British Museum scholars first learned from cuneiform tablets that the Babylonians had a flood myth, they were disturbed by its striking similarities to the story of Noah.
"Already in 1872 people were writing about it in a worried way — What does it mean that Holy Writ appears on this piece of Weetabix?" he joked, referring to a cereal similar in shape to the tablet.