Then - ("And") Jonah prayed, i. e., when the three days and nights were passed, he uttered this devotion. The word "prayed" includes thanksgiving, not petition only. It is said of Hannah that she "prayed" 1 Samuel 2:1; but her canticle is all one thanksgiving without a single petition. In this thanksgiving Jonah says how his prayers had been heard, but prays no more. God had delivered him from the sea, and be thanks God, in the fish's belly, as undisturbed as in a Church or an oratory, secure that God, who had done so much, would fulfill the rest. He called God, "his" God, who had in so many ways shown Himself to be His, by His revelations, by His inspirations, by His chastisements, and now by His mercy . "From these words, 'Jonah prayed unto the Lord his God out of the fish's belly,' we perceive that, after he felt himself safe in the fish's belly, he despaired not of God's mercy."

Jonah 2:1 Parallel Commentaries

Whether the Fire of Hell is Beneath the Earth?
Objection 1: It would seem that this fire is not beneath the earth. For it is said of the damned (Job 18:18), "And God shall remove him out of the globe [Douay: 'world']." Therefore the fire whereby the damned will be punished is not beneath the earth but outside the globe. Objection 2: Further, nothing violent or accidental can be everlasting. But this fire will be in hell for ever. Therefore it will be there, not by force but naturally. Now fire cannot be under the earth save by violence. Therefore …
Saint Thomas Aquinas—Summa Theologica