Many Waters — Madeleine L'engle

A touch of computer keys, a blast of heat, and suddenly the Murry twins, Sandy and Dennys, are gasping in a shimmering desert land. If only the brothers had normal parents, not a scientist mother and a father who experiments with space and time travel. If only the Murry twins had noticed the note on the door of their mother's lab: EXPERIMENT IN PROGRESS. PLEASE KEEP OUT.

But it's too late for regrets. There's a strange -- and very small -- person approaching, with a miniature mammoth in tow....

At last it's Sandy and Dennys's turn for an adventure -- an adventure that turns serious when they discover that "many waters" are coming to flood the desert. The twins must find a way back home soon, or they will drown. But how will they get back to their own time? Can they?