Stranded (The Prometheus Project, 3)

From the New York Times and USA Today best-selling author of the adult thriller Wired, Stranded is the third book in The Prometheus Project, a critically acclaimed science fiction series for kids.
A fantastic alien city buried deep underground. What wonders - and what dangers - is it hiding?
Ryan and Regan Resnick are the two youngest members of the Prometheus Project, a top secret team exploring a vast, abandoned alien city. But when they journey through a portal to a primitive alien planet, what begins as an ordinary day will quickly become a nonstop fight for their lives.
Soon they are stranded on the distant planet, surrounded by vicious predators and in the path of a raging river of lava. But surviving on the deadly planet might be the easy part. Because if they can get back to Earth, they will have to face a ruthless adversary who controls a mysterious alien device. A device that is the most powerful, dangerous, and unstoppable weapon the world has ever known.