What's this book called...?

It's this book series that i use to read in fifth grade but i can't remember the name. It was about a guy who found a tunnel to the underworld and he was the one all their prophecies were about. Their saver or something. And his dad had been missing for years and it turned out he was trapped down there. Then in one book his little sister ends up down there so she has to come with him. Its for elementary schoolers. Does anyone know what its called? I them his name was greg or gregory