The mouth spews forth - some say for leagues forty
And from it's gate it is said the faithful did sorty
NOTE: Bay of Romenna was the incorrect answer to this riddle.
The Bay of Rómenna was an inlet of Belegaer on the eastern side of Númenor that separated Orrostar to the north from Hyarrostar to the south.
Tolkien Gateway: Bay of Rómenna