Until modern times people thought the ocean floor was sandy like the desert and saucer shaped-deepest in the middle. This was even true of the pre-1900 geologists.
But in the 1900s oceanographers found the sea had many deep valleys or canyons. The deepest canyons were called trenches. The Marianas Trench in the Pacific is so deep that if Mt. Everest (29,000 feet high) was dropped into it, the peak would still be a mile below the water’s surface.
There are also underwater mountains. The Atlantic Ocean contains an undersea range of mountains 10,000 miles long.In addition, 3,000 years ago the Bible spoke of the valleys and mountains of the sea. In Psalm 18:15 (NIV) David wrote of God being the creator of "the valleys of the sea." God asked Job (38:16 NIV): "Have you walked in the recesses [valleys] of the sea?" The prophet Jonah was thrown off a ship and spoke of falling to the bottom of the mountains in the sea (Jonah 2:6).
The Bible spoke of the valleys and mountains of the sea thousands of years before scientists discovered them. Indeed our Bible is the inspired Word of God!