Captured by a Dragon by Christina Wilder

If my two best friends hadn’t dared me to go spelunking, I wouldn’t have drunk too much celebratory champagne afterward, and I might not have fallen into a mysterious, dark tunnel in the back of a cave. In seconds, I’m sliding down, down, down, for a very long time. I somehow lose a few bits of clothing along the way. Major embarrassment! I shoot out of the tunnel, launch through a waterfall, and land in the lap of a gorgeous, muscular, naked guy. My…Well, you know, pressed against his…Well, you know. Talk about a compromising position!
One touch leads to another, and before I know it, I’m exploring a different compromising position with this hunky man. I’m falling for him fast, but there are a few odd things about him. He talks like he’s old-fashioned. He calls me a surface dweller. And he insists I’m his destined mate. A dragon’s destined mate.
Now, I have to decide if I want to escape his fiery embrace and return to the world above, or remain with him forever, deep in the center of the Earth.