Archeologists exploring the site of a planned 32-story office tower announced Thursday that they have unearthed remains of a Roman basilica that once stood at the heart of the city known as Londinium.

An office building from the 1930s in London was about to be demolished — until remnants of the city’s first Roman basilica were found 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.