Many Waters is a 1986 novel by Madeleine L'Engle.

The principal characters of the story are Alexander (Sandy) and Dionysius (Dennys) Murry, twin brothers who are, paradoxically, somewhat out of place (i.e. for being "normal") in the context of the multifarious and eccentric Murry family from A Wrinkle in Time. The action of the story follows that of A Wind in the Door but precedes the climactic events of A Swiftly Tilting Planet.

In the middle of a New England winter, the boys accidentally disturb an experiment in their parents' lab. A sonic boom - a wave of mist - and the boys find they have been transported to a vast, trackless desert which is shaken by periodic earthquakes. Providentially, they encounter a water prospector named Japheth who offers to help them find refuge at the nearest oasis. Sandy and Dennys are intrigued by the creatures which accompany them on their trip through this (as they initally assume) alien world, which include a two-foot-tall mammoth and a pair of unicorns which appear simultaneously to be, and not to be.

After a long ride through the desert during which they develop a severe case of heat stroke, the boys are suddenly and mysteriously separated. Sandy remains with Japheth and his elderly grandfather Lamech and is tended to by a variety of improbable beings, including a pelican. Dennys makes his way to a large tent in the center of the oasis and comes under the care of a friendly family headed by a gruff but kindly patriarch. As he recovers from his "sun-sickness", Dennys learns that his benefactor is in fact Japheth's father and Lamech's son - and his name is Noah... It soon becomes apparent that the boys have been transported back to biblical times, just before the Great Flood. As well as humans there are also two races of angels: The heavenly seraphim and the main antagonists of the story, the fallen, beautiful mortal marrying nephilim (the fallen angels) The story largely concerns the teenaged twins' emotional coming of age in the antediluvian milieu, but, like the other three novels about the Murry family, includes elements of fantasy and Christian theologysuch as two races of angels: The heavenly seraphim and the main antagonists of the story, the fallen, beautiful mortal marrying nephilim (the fallen angels) . Similarities to the fantasy-science fiction works of C. S. Lewis, always present in L'Engle's oeuvre, are particularly notable here.