Hypogeum of Ħal-Saflieni
Location: Malta. Built: Around 2,500 B.C. Unlike the other temples on our list, the Hypogeum in Malta was constructed underground. A UNESCO World Heritage Site, this enormous, subterranean labyrinth has false windows, trilithon doorways, decorative red ocher paintings, and carved-stone ceiling accents that mimic corbeled masonry. The Hypogeum was discovered in 1902 during construction and was first excavated beginning in 1903. The site was closed between 1990 and 2000 for conservation, and while it has reopened to the public, only 80 visitors are permitted per day.