Facilis descensus Averni;
Noctes atque dies patet atri janua Ditis;
Sed revocare gradum, superasque evadere ad auras,
Hoc opus, hic labor est.

The descent to Avernus is easy;
the gate of Pluto stands open night and day;
but to retrace one's steps and return to the upper air,
that is the toil, that the difficulty.

The Aeneid, Virgil