Lake Marathon is an artificial reservoir created in the 1920s by damming the Haradros river in the mountains above the Marathon plain. The dam itself is one of the finest examples of early 20th-century hydraulic engineering in Greece — faced in Pentelic marble, the same white stone used for the Parthenon, making it one of the most handsomely dressed dams anywhere in the world. The reservoir supplied Athens with water for decades and continues to serve the city.
The lake is set in hilly country with pine forest on the slopes above the water, and the drive up from Marathon provides views across the reservoir to the mountains of northern Attica. The marble-clad dam face is visible from the road above. Swimming is not permitted in the reservoir as it is still used as a water supply, but the area makes a good complement to the battlefield sites — the contrast between ancient history in the plain below and the 1920s engineering works above is part of what makes Marathon an unusual place to visit.