Albert Barnes Commentary Amos 9:3

"And though they hide themselves in the top of Carmel, I will search and take them out thence; and though they be hid from my sight in the bottom of the sea, thence will I command the serpent, and it shall bite them." — Amos 9:3 (ASV)

He had contrasted heaven and hell as places impossible for humans to reach; as David says, If I ascend into heaven, You are there: If I make my bed in hell, behold You (Psalms 139:8). Now, concerning places in a way accessible, he contrasts Mount Carmel, which rises abruptly out of the sea, with the depths of that ocean which it overhangs. Carmel was a hiding place in two ways.

1. Through its caves (some say 1,000, some 2,000) with which it is perforated, whose entrance sometimes scarcely admits a single person; so close to each other that a pursuer would not discern into which the fugitive had vanished; so serpentine within that, as a traveler says, “10 steps apart, we could hear each other’s voices, but could not see each other”: “Carmel is perforated by a hundredfold greater or lesser clefts. Even in its appearance of loveliness and richness, the majestic Mount, by its clefts, caves, and rocky battlements, evokes in the wanderer who sees them for the first time, a feeling of mingled wonder and fear. A whole army of enemies, like nature’s terrors, could hide themselves in these rock clefts.”
2. Its summit, about 1800 feet above the sea, “is covered with pines and oaks, and lower down with olive and laurel trees.” These forests furnished hiding places for hordes of robbers at the time of our Lord. In those caves, Elijah was probably hidden at times from the persecution of Ahab and Jezebel. It seems to be spoken of as his abode (1 Kings 18:19), and also as one of Elisha’s resorts (2 Kings 2:25; 2 Kings 4:25). Carmel, as the western extremity of the land projecting into the sea, was the last place a fugitive would reach. If one found no safety there, there was none in the whole land. Nor was there safety by sea;

And though they be hid—(rather, “hide themselves”)—from My sight in the bottom of the sea, thence will I command the serpent. The sea too has its deadly serpents. Their types are few, but the individuals in those types are much more numerous than those of land serpents. Their shoals have provided sailors with signs of approaching land. Their chief habitat, as traced in modern times, is between the Tropics.

The ancients knew of them perhaps in the Persian Gulf or the Red Sea. All are described as “highly venomous” and “very ferocious”: “The virulence of their venom is equal to that of the most pernicious land serpents.” All things, whether by their will or without it through animal instinct (like the serpent) or their savage passions (like the Assyrian), fulfill the will of God. Just as, at His command, the fish that He had prepared swallowed Jonah for his preservation, so, at His command, the serpent would come forth from the recesses of the sea to cause the sinner greater suffering.