Tom Swift and His Undersea Search, Or, The Treasure on the Floor of the Atlantic, is Volume 23 in the original Tom Swift novel series published by Grosset & Dunlap.
A Mr. Dixwell Hardley approaches Tom with a proposition to help recover sunken treasure. Mr. Hardley was on board a ship which was carrying gold to help finance an illegal revolution. When the ship sank, Mr. Hardley overheard the captain recording the coordinates. Now he wants Tom's help to recover the gold, under the guise of both financing the expedition as well as rewarding Tom with a portion of the recovered treasure.
Unfortunately for Tom, after agreeing to the expedition, he learns that Mr. Hardley is a con-artist, who recently scammed someone out of the oil well rights. Making matters worse, the victim is Barton Keith, a relative of Mary Nestor. Rather than cancel the expedition, Tom decides to carry on in the hopes of restoring Mr. Keith's claims to the oil wells.