An office building from the 1930s in the heart of London was about to be demolished and redeveloped by its owners — until archaeologists unearthed remnants of the city’s first Roman basilica in the basement.

Experts are surprised at how much of the basilica, or town hall, survived. The ancient building was part of Roman London’s political and commercial center.

The 2,000-year-old basilica was “once the beating heart of Roman London,” the Museum of London Archaeology said.