I bought this book in the 6th grade in 1972. The story is about a teen and his college brother who are orphans. It starts with them filming a tsunami, or other big wave, hitting the coast where they live. As they climb down the cliff face after the wave, the older one finds a statue of a bipedal alien figure embedded in the cliff face now exposed by the destruction of the wave. The statue appears to be made of a diamond like substance. Their exploring leads them to a chamber under the sea with a coffin like box similar to the statue's material containing a body like the statue. With the help of a professor the box is moved to the university and the body is accidentally revived. The being turns out to be from a civilization millions of years in the past. The story unfolds as they travel with the guy to his city, then on deep into the planet searching for the remnants of his people.
Stranger from the Depths by Gerry Turner
From this review:
In a nutshell, 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. He -- having been in suspended animation for eons -- leads them to his still-extant city, millions of years old, but protected by various autonomous systems and its diamond-like outer shell. It is a marvel to our contemporary travelers, who (through the "induction machine") learn to communicate with Saa. Saa, naturally, is feeling a bit lonely -- until he finds that there are others of his race in a different, renegade city that is reaching the end of its lifetime. Its people have become superstitious, and unable to wield science and engineering any longer, they rule through a priesthood-like theocracy. Saa and his human friends come in peace, in an attempt to merge the two cities and its peoples...