The sanctuary of the Great Gods on the Greek island of Samothrace was home to one of the ancient world’s most renowned mystery cults. The kings and queens who vied to succeed Alexander the Great funded lavish building projects at the remote island sanctuary where Alexander’s parents had first met. Currently, the American archaeological excavation on Samothrace is investigating the sanctuary’s largest building, a 100-meter-long portico dubbed the Stoa. Hidden behind the Stoa’s austere Doric façade we have discovered the earliest case of flat arches, a construction technology widely used in the Roman empire as well as the modern world.