CAIRO — Jet black and glistening with mud, the giant granite sarcophagus sits at the bottom of a pit in the Egyptian city of Alexandria, perfectly sealed despite being 2,000 years old, prompting ever-wilder theories about what secrets might lie inside.

The discovery of the sarcophagus on a construction site last month was a rare find in Alexandria, a fabled port city where most traces of Egypt’s ancient civilizations have crumbled into the waves or lie buried beneath urban sprawl.

A contractor digging foundations for a building on Al Karmili Street spotted the gleam of the burial vessel, which measures nearly nine feet by five. Archaeologists continued the dig by hand, and were excited to discover that the mortar seal around the heavy lid was entirely intact.

That is unusual: Centuries of plunder by treasure hunters and professional tomb raiders have spoiled many ancient Egyptian burial sites.