pay dirt

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

• noun Earth, ore, or gravel that is profitable to mine.
• noun Informal A useful or profitable discovery or venture.

from the GNU version of the Collaborative International Dictionary of English.

• (Mining), Western U. S. Earth, rock, etc., which yields a profit to the miner.
• a discovery, especially after an exploratory process, which yields a profit or sought-after benefit; -- used especially in the phrase “to hit pay dirt”.

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

• noun mining Earth which contains profitable quantities of ore.
• noun figuratively A profitable area or period; success.

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

• noun a profitable success
• noun ore that yields a substantial profit to the miner