The Temple of Valadier, or Il Tempio del Valadier, is a perfectly octagonal church built inside a cave in Italy. It was commissioned by Pope Leo XII in the 19th century.
The church has a domed roof and eight sides that are meant to symbolize Jesus' resurrection "after eight days." Today, it's known as the "Refuge of Sinners" and has long been a site of pilgrimage for Catholics seeking forgiveness.