Captured (The Prometheus Project, 2)

Loved by kids and adults alike, Captured is the second book in a critically acclaimed science fiction series from New York Times and USA Today best-selling author Douglas E. Richards.
Ryan and Regan Resnick are the youngest members of a top-secret team exploring the greatest discovery ever made: a vast alien city buried deep underground - as potentially deadly as it is astonishing. When the city is captured by highly trained soldiers led by a ruthless alien, the adult members of the team are taken hostage. Now Ryan and Regan are the team's only hope of survival.
With the future of the world at stake, the Resnick kids must do the impossible: outwit the brilliant alien, free the prisoners, and thwart an unstoppable invasion. But not everything is as it seems. And time is quickly running out.