Archaeologists excavating a construction site in South London have discovered a Roman-era mosaic floor thought to be at least 1,800 years old — a “once-in-a-lifetime” find that one said is the “largest area of decorated mosaic discovered in London in the past 50 years.”

The colorful double-paneled mosaic was uncovered by the Museum of London Archaeology (MOLA) as part of ongoing excavations of an area in Southwark that will be the site of a multiuse development known as the Liberty of Southwark.

MOLA conservators are expected to be on the site starting this week to assess the mosaics and later transport them to the museum for conservation. The long-term plan is for the mosaics to be put on display.

Antonietta​ Lerz, a senior archaeologist at MOLA who supervised the excavation with her colleague Dave Saxby, described the mosaics in an email to The Washington Post as a “once-in-a-lifetime” find because of their “size, rarity and preservation.”

A news release announcing the discovery on Tuesday said the largest of the two mosaic panels is from the late 2nd to early 3rd century, while the room it was in “was clearly in use for a longer period of time.”

“Astonishingly, traces of an earlier mosaic underneath the one currently visible have been identified,” the release says. “This shows the room was refurbished over the years, perhaps to make way for the latest trends.”

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