MILAN, Italy — One of the most ancient churches of Venice, a Byzantine basilica established in the year 639, counts among the 60 churches damaged in three exceptional floods last week, officials said Tuesday.

The ancient Santa Maria Assunta Basilica, and the adjacent Santa Maria Fosca church, were “abundantly flooded” three times last week, with the lagoon salt water seeping into mosaic floors and the marble columns, said Alessandro Polet, spokesman for the Venice Patriarchate said.

The basilica and its mosaic floors have been cleaned with fresh water, but the extent of the damage will take time to assess. “Damage from salt water you only see after time,’’ Polet said.

Because the salt water penetrates the building materials, damage is often much higher and deeper than the actual water levels. Due to its position in the lagoon, the water took longer to recede than from the historic center of Venice.

The floods also destroyed a walkway to the nearby bell tower, Polet said.

Venice city hall noted the damage on Instagram, saying the basilica, “a container of priceless treasures,” had been “hard hit.”

The basilica, which underwent renovations in 864 and 1004, contains the earliest remaining mosaics around Venice.

The stone floor mosaics incorporate geometric circular and checkerboard patterns, while the main apse features an 11th century mosaic of the Virgin Hodegetria — an iconographic depiction of the Virgin Mary — against a golden background, hovering above a line of saints.

The Venice Patriarchy estimates half of Venice’s 120 churches sustained some level of damage, with a preliminary average estimate of $66,500 each — not taking into account the city’s centerpiece, St. Mark’s Basilica.

In addition, cultural officials said 13 of Venice’s 80 bell towers were being closely monitored in the wake of the floods for structural damage.

St. Mark’s crypt was under water for a full 24 hours, having flooded for only the second time in history, after 1966, according to Carlo Alberto Tesserin, the first procurator, charged with preserving the basilica.

While damage to the crypt is expected to be extensive, Tesserin said it was too early to estimate. The basilica, which sits at Venice’s lowest point, was also battered by an exceptional flood in October 2018, with damage to the bronze metal doors and columns.

“The floods can come back at any moment,” Tesserin said. “The forecasts are no longer reliable, due to the changing climate.”

Antonia Pasqua Recchia, an official in the Italian Cultural Ministry, told Sky TG24 that it wasn’t enough to restore the damaged treasures and that “this needs to be about prevention.”

Toward that end, city officials are pressing Rome for completion of long-delayed movable underwater barriers, which are meant to protect Venice from floods above 1.1 meters (3 feet, 6 inches). Last week’s floods measured 1.87 meters, 1.54 meters and 1.5 meters — the first time three such exceptional floods have hit the City of Canals in recorded history.

Beyond the churches, Pasqua Recchia said, “we should also remember the museums and the palaces that must confront a cleanup that will be long and costly.”

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