Two stone columns flank the grand entrance to St. Mark’s Square. They’re among the first sights visitors see coming in from Venetian Lagoon off the Adriatic Sea. One features a winged lion, the symbol of St. Mark. The other holds a statue of St. Theodore, once Venice’s patron saint, standing over a dead crocodile.

The columns were erected in the mid-13th century, when the water was much closer to the square. Gambling was allowed in the space between them, and public executions were carried out there, in front of the lagoon. Now the area is filled with tourists who are likely unaware of one discrepancy: There should be three columns.

The columns arrived in Venice towards the latter half of the 12th century—more than 90 years before they likely went up. There were originally three on the ship that came in off the lagoon; one sank on the way in and no one is sure where. Worse, if it sank somewhere outside the area in front of St. Mark’s, where the lagoon’s floor is thick clay, it’s likely gone forever, swallowed up in the muddy bottom.

In 2016, researchers were given the go ahead to search for the third column, about which almost nothing is known. Is it the same size? Does it feature a statue at the top? The answers are hidden with the stone pillar. For now, one can visit the site where it could be, to admire the two surviving columns and maybe even mourn for the third, gone too soon and for too long.