The emerald tablet

"When Gabe Conway and his friends find a strange old map in his uncle's office and follow it to a crumbling secret chamber beneath San Francisco, they think they're just having one last adventure before Gabe moves away. They don't expect to end up bound to the magic of the elements, or to set off a chain of events that none of them can explain. But they're about to get more of an adventure than they bargained for. For the last century, a power-hungry cult has been plotting to merge our world with a twisted parallel realm. Gabe's bloodline holds the key to the barrier between the worlds--and now it's up to Gabe, his friends, and their new elemental powers to save San Francisco from utter destruction"--Amazon.