This was a book I read in the ‘70’s about a deserted futuristic city under the ocean floor that appears to be abandoned but is discovered by two teenaged scuba diving boys. They find a statue in an underwater cavern of an amphibious man. Things get blurry here, but somehow they awaken one of the inhabitants of the city and use one of two drilling machines called Mole One and Mole Two to find an even deeper civilization inhabited by a bad guy amphibian.
The statuette, the teenage boy protagonists, and the drilling machine makes it sound like Stranger From the Depths by Gerry Turner, published 1967 and widely distributed as a Scholastic paperback in the US. Quoting this description on Goodreads,
. . . we follow a band of high school friends who've discovered a statue made of diamond after the New England coast is hit by a tsunami. This -- along with a college professor mentor of theirs -- leads them to discover an undersea tomb which contains Saa, the lizard man depicted on the cover
(Contra that description, I do believe it's a duo rather than a "band" of friends.)
Does this cover look familiar?