divining rod

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A forked branch or stick that is believed to indicate subterranean water or minerals by bending downward when held over a source.

from The Century Dictionary.

• noun A rod or twig used in divining; especially, a twig, generally of hazel, held in the hand and supposed by its bending downward to indicate spots where metalliferous deposits or water may be found by digging.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun rod used for dowsing, a technique of divination used to locate subterranean sources of water, metal, other mineral resources or even various other things through magic, or according to many believers a natural phenomenon

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun forked stick that is said to dip down to indicate underground water or oil