"The Boxes", by William Sleator

The Boxes. It's by William Sleator, who also wrote several other creepy books (which are also excellent sci-fi reads).

It begins with Uncle Marco, Annie's uncle (obviously), leaving for some mysterious destination. But before leaving, he entrusts Annie with two strange boxes, which he warns her must be separated from each other. She complies, carting the disproportionately heavy wooden box to the basement, and the smaller metal one up to her room, to be stored in the back of her closet. With that, and a final order to keep the boxes away from her greedy, self-centered aunt, he leaves.

Soon after Uncle Marco's departure, Annie is seized with an overwhelming curiosity. Going against all of her uncle's warnings – how bad could the boxes be, anyways? – she pries open the wooden box downstairs...

... And a strange metal creature scuttles out, running to hide away in the darkness of her basement.

Annie runs. But the crab-like creature can't keep her away – she goes down to investigate, and discovers – oh, horror of horrors – that it has made another being like itself. Through telepathic communication with the first thing, Annie agrees to go up to her room, and open the second box....

Strange happenings commence. Time slows, a corrupted company is buying out the neighborhood, and the creatures in the basement begin to build a city. Annie becomes a messenger to the second box, a clock-like thing, and is forced to sacrifice smaller, less-important members of the strange society downstairs to make time slow.

But the two kids aren't the only ones to figure out the connection. The company wants the second box, and is willing to do (almost) anything to get it...

In a perfect swirl of suspense, mystery and nearly unimaginable alien-esque happenings, Sleator has made a great book with a Pandora-like theme (Annie being Pandora). Of course, I was in agony at the ending. Unfortunately, he has not written a second book to tell what happens next.