Sounds like the story in this other question.
The name of the book is "Stranger from the Depths." It was first published in 1967, with a paperback edition in 1970.
The author (Gerry Turner) is actually an American.
I read this book also. Long ago. Like, 1970s.
The character you are thinking of is named Saa.
I forget how he was found, but he isn't the only survivor of his species.
They had some kind of force field technology that let them build cities under the ocean, and had some kind of vehicle that could pass through solid rock. The outside of the vehicle would get hot, and the whole thing melted its way through rock.
Some of the other survivors were some kind of bad guys. The kids and Saa some how ended up taking a trip through rock to get something (I want to say they needed some special crystal or mineral to rejuvenate one of the city force fields.) The group of other survivors goes along, but in a second vehicle.
Saa had repaired both vehicles, and had already suspected the other group were bad guys.
On the trip, the bad guys make clear that they are the bad guys. On the way back, Saa takes a little detour through a slightly tougher section of rock. The bad guys are following because they don't know how to navigate through rock.
On their little detour, Saa's vehicle has to raise the heat output to full power to get through the rock. While this is going on, the kids are worrying about what they are going to do. The bad guys have the Mcguffin, and that will give them a lot of power in our world.
Saa brings the vehicle back to the easier course and tells the kids to relax. When he fixed the other vehicle, he sabotaged it so that if it ever went to full power, something important would burn out. Thanks to the little detour, that had happened and the bad guys were now trapped under a couple of miles of rock.
I thought it was a pretty good story back then, but I was like 10 years old. I haven't read it since. It might not be as good as I remember it. :)
