Gregory and the gray prophecy

Beneath the streets of New York, your greatest adventure begins.
When eleven-year-old Gregor tries to save his little sister from falling down a ventilation shaft, they fall into a seemingly endless void. They land in the Underland, a world beneath New York City. There, almost transparent-looking humans live alongside giant talking cockroaches and bats. Gregor is fascinated, but still wants only one thing: to get back home as soon as possible. It's enough that his father disappeared without a trace more than two years ago! That's when Gregor learns that he's in a prophecy. If he gets involved with it, he could not only help the Underlanders, but also find his father.