This magnificent piece of work is unique in the world because its external cladding of white pentelikon marble is the same marble used in construction of the Parthenon and the other buildings in the Acropolis. Works for the dam started in 1929. The man-made lake was the main water supply for Athens from 1931 to 1959. Now it serves as a secondary supply.
It's 5 minutes drive from Marathon village and the site of the historic battle of Marathon where the Greeks defeated the Persians. The Marathon Race name originates from that battle. See here why.
Cars can drive through the top of the dam.
In greek, Marathon means fennel.
See the site on the map