Peisistratus built the first aqueduct tunnel at Athens, which most likely had its sources on the slopes of Mount Hymettos and along the Ilissos river. It supplied, among other structures, the fountain house in the southeast corner of the Agora, but it had a number of branches. In the 4th century BC it was replaced by a system of terracotta pipes in a stone-built underground channel, sometimes called the Hymettos aqueduct; many sections had round, oval or square access holes on top of about .