The Dark Water (Well's End Series #2)

To escape Blake Sutton’s army at the end of the enthralling The Well’s End, Mia and her friends jump into the newly gurgling fountain of youth and swim to its very bottom. When they resurface, an astounding world awaits them—an entire underground civilization of humans, the Keepers of the Well.
 
But instead of finding a safe haven, Mia and her gang are quickly embroiled in a dangerous, high-stakes battle royale. If Mia wants to save everyone she loves and make it back home alive, she’s got to get to the water’s Source before Sutton and his troops, who are still hot on her trail.
 
With new characters and new threats, Seth Fishman has upped the ante fantastically and delivers another tense, fast-paced adventure in a richly imagined world just below our feet.