Undermountain by Eric Kent Edstrom

The Undermountain Saga
Perfect for fans of YA adventure and science fiction. A group of teens embarks on a backpacking adventure, each wanting an escape from real life, each for a different reason. For Danny it's his mother's domineering boyfriend; for cheerful Breyona, it's a suppressed memory; for Shiv it's a tragedy he keeps secret; for Wa it's a chance to meet girls.

They have no idea that their hiking trip will take them farther from home than anyone has ever gone before!

Undermountain (Book 1)
Danny Michaelson joins a youth backpacking expedition into the Canadian Rocky Mountains. He's immediately captivated by the beauty of the mountains--and the alluring charm of Breyona Lewis.

But on day six, the journey becomes the adventure of a lifetime when Danny and Breyona encounter a creature of legend. Bigfoot. And it speaks to them.

Swept to the secret, high-tech city of Undermountain, Danny, Breyona, and the others find themselves caught in the middle of a war between two alien races . . . one with plans to destroy the Earth, the other with plans to rule it.

The Undermountain Saga continues in Afterlife and concludes in Starkiller.