Marco's Millions

Description for MARCO'S MILLIONS by William Sleator:

A door to another world

When Marco's fragile little sister, Lilly, tells him about the strange lights in the basement, he thinks she's imagining things. But Marco is curious, and it isn't long before he investigates and finds a passage into a strange world, the likes of which he's never seen. The insect-like inhabitants who live there know all about Lilly and believe she is special. They beg Marco to bring her to them so she can save their world from ruin. Can Marco kelp the creatures and protect Lilly from harm?