The remains of an ancient basilica that would have formed the heart of Roman London have been uncovered during building work in the center of the British capital.

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.

Archaeologists have found “the heart of Roman London,” the remains of the city’s first basilica, underneath an office building.

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.