London Mithraeum sits in the basement of Norman Foster's Bloomberg Building on Walbrook, seven metres below modern street level. The temple was built around 240 AD beside the River Walbrook and dedicated to the Persian-born god Mithras, whose secretive cult required terrifying initiations and bound its members to silence. Only men could join, and the windowless interior mimicked the cave where Mithras supposedly slew a sacred bull. Lost for centuries, the temple was discovered by chance in 1954 when the bomb-damaged site was being redeveloped, causing a public sensation. It was moved stone by stone, then returned close to its original position when Bloomberg's headquarters were completed in 2017.

VoiceMap's self-guided audio tours trace the cult's journey from ancient Persia to Roman Londinium, connecting the Mithraeum to the lost River Walbrook, the city's subterranean Roman layer and the curious parallel between Mithras's birth date and Christmas Day.