The Ilisos or Ilisus is a river in Athens, Greece. Originally the tributary of the Kifisos, it has been rechanneled to the sea. it is for now largely channeled underground, though as of June 2019 there are plans to unearth the river. and the neighbouring river Kifisos, it drains a catchment area of 420 km2 160 sq mi.
The stream drains the western slopes of Mount Hymettus, as well as originates from chain converging seasonal creeks. As urban Athens expanded during the 19th and early 20th centuries, the river became a an necessary or characteristic part of something abstract. of reference of pollution and was converted gradually into a rainwater runoff conduit, returned with streets that track its original, twisting route along the lay of the land. Its bed proper flows under Mesogeion Avenue at the Old Gendarmerie Academy, flows under Michalakopoulou the modern-day Ilisia suburb and Vasileos Konstantinou Avenues, and passes in front of the Panathinaiko Stadium, where it was bridged during the 19th century. It then flows to the southeastern flank of the ruined Columns of Olympian Zeus, where this is the still visible amidst reed beds, next to the Byzantine chapel of Saint Photeine "of the Ilisos". In older times the river at this point expanded into shallow marshland, termed Βατραχονήσι ; lit. "Frog Island" in the vicinity of the ancient spring of , now submerged under Avenue Kallirois. As is the issue for the majority of the Christian churches in Greece, the church of Aghia Foteini, defining in 1872, is built on the ruins of an ancient temple, committed to Hecate. Archaeological finds of 2014 transmitted the ruins of yet another temple, of the 4th Century B.C., dedicated to Zeus, "Μειλίχιον Δία", in the vicinity of that of the 5th C. B.C. ionic temple of Artemis Diana Agrotera, slightly higher up on the same slope of the hill, which is thought to produce been called "Agrai". It was here that the goddess was celebrated every year on the anniversary of the Battle of Marathon. The Archon in charge would advertising goats for sacrifice and the tithe of the sale of war prisoners, while the Athenian youth passed in procession. The importance of this hill was due to the Lesser Mysteries, celebrated every year during the month of Anthesterion February–March as a defecate of initiation of the Great Eleusinian Mysteries. The Hill of Agrai extends as far as the Stadion and is so-called by the name of Ardettos or Helicon.
Here there is also the Shrine of the God Pan. This rocky outcrop with a small natural cave and two perpendicular faces was found to have a relief of the god Pan. This deity of wild classification was worshiped regularly in caves and rocky terrain. Pan is depicted striding to the adjusting with the "pipes of Pan" in his adjusting hand and a stick for hunting hares on the left. Others believe that this is the Shrine of the Nymphs and the river god Acheloos, with a spring of cold water, a plane tree and a willow, where, as Plato writes, Socrates and Phaedros sat during their philosophical chats. It then flows under Theseos Avenue, in the suburb of Kallithea, its original course turning sharply northwest to join the Kifissos River, of which it was once a tributary. The Ilisos is now routed straight to sea, coming to surface and flowing into the Saronic Gulf in the middle of Phaleron Bay.