The plan was to demolish the old office building at 85 Gracechurch Street in the heart of London’s financial district and construct a new 32-story tower in its place. But as demolition began, archaeologists made a staggering discovery in the site’s foundation: remarkably intact ruins of London’s first public building,

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.

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