Cavern of the Shining Pool by Arthur Leo Zagat

Jay Silton returned to Earth looking like the 20-year-old he had been when he left aboard the spaceship Luna 30 years ago—a spaceship that had been lost with all hands aboard.

Now Jay had a strange tale to tell of another world in a different universe. A world of monsters. There, time ran differently, and while he had been held captive by a strange race of turtle-like aliens, 30 years had passed on Earth...but it had only been a month for him!