When hence your destin'd voyage ye pursue,
Two rocks will rise, tremendous to the view,
Just in the entrance of the watery waste,
Which never mortal yet in safety past:
Not firmly fix'd; for oft with hideous shock
Adverse they meet, and rock encounters rock:
The boiling billows dash their airy brow,
Loud thundering round the ragged shore below.

Lines 436–443 - Argonautica (3rd century BC) - Book II