Older Sci-Fi about young brother and sister exploring an ancient domed city under the ocean.

[jpdejongjr]

I read this paperback book in the 1970s. It had a blue-green cover that showed the domed city underwater. The kids had a father that was the scientist exploring the city. The kids found a chair that inserted a wire in their head and taught them the ancient language. The kids revived an ancient alien being. They used a mole machine to travel through the earth to search for others of the alien's kind that may have survived.

This book really got me started reading sci-fi as a teenager. It felt like a book for young adults, but it was a full paperback book, not a short story. I remember the pages were a bit yellowed, so this book could have been published in the 50s or 60s?

 

[otistdog]

I haven't read it myself, but the plot of Stranger from the Depths (1967) by Gerry Turner seems to agree in broad outline. As described in a goodreads.com review by user Ravenpi:

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 -- and I won't give away any more.

 

[TomMazanec]

The cover had the lizard man

 

[jpdejongjr]

This might be it. I'll have to find a copy and give it a read. It was so long ago, the details are covered with so much dust in my memory...

Thank you to all that looked into this!