Elkiron - Elkiron Series Book 1 #775075

A secret cave lies hidden in the mountains of Montana. Unseen by human eyes for centuries, until two children stumble upon it. They discover the ruins of an ancient city deep inside the cave. They try to return a second time but they can't find the entrance. Bryce, the youngest child, learns how to find the cave and leads his family to it. 
Chelsea is an archaeologist and is overwhelmed by the ancient city. As they explore they find writing in a language unlike any found on Earth. After further exploration of the city, they find the key to understanding the language and learn that the city was built long ago by Elves!
They discover a newer, sealed section of the cave near the ancient city. While exploring, they find themselves face to face with a living Elf!
He explains the history of the ancient city and asks them to help him save his home planet, Elkiron. While there, they discover the secret of their own ancestry and help the Elves fight  
a war against the dark magic that is taking over the planet.