The tunnel runs under Mount Kastro for a kilometre, cut through limestone by hand in the sixth century BC to bring water from a spring on the north side of the mountain to the ancient capital on the south. It served the city for a thousand years and remained one of the great underground works of the ancient world.
Visitors can walk a long section of it on a wooden walkway, ducking under the rock ceiling and seeing the original aqueduct channel three metres below — an extraordinary thing to be inside. Tickets are limited; come early or book in advance during the high season. The tunnel is north of Pythagoreio and a short drive from the town.