A fighter pilot crash-lands on an island and is sucked into a whirlpool of sand, finding a civilization deep inside the Earth. He is imprisoned there and manages to escape with a few others after a long time. When he describes what happened, no one believes him.
Wild shot: The Secret People by John Wyndham (Writing under his pen-name, John Bunyan) https://en.wikipedia.org/wiki/The_Secret_People