In the Bosphorus the Black Sea flows always into the Egean sea, and
the Egean sea never flows into it. And this is because the Caspian,
which is 400 miles to the East, with the rivers which pour into it,
always flows through subterranean caves into this sea of Pontus; and
the Don does the same as well as the Danube, so that the waters of
Pontus are always higher than those of the Egean; for the higher
always fall towards the lower, and never the lower towards the
higher.

Constantinople.