mother lode

Originally meaning a main vein in mining, the spelling “motherload” is a mistake which is probably influenced by people thinking it means “the mother of all heavy loads.”

A “lode” was originally a stream of water, but by analogy it became a vein of metal ore. A vein of mineral ore deposited between clearly demarcated layers of rock ahdictionary dot com Miners of precious metals live to find a 'lode' of gold, which they refer to as a “mother lode,” often spelled as two words, sometimes as one (neither is in the dictionary though).

Definition paraphrased from Washington State University website

Son of a bitch, we struck the mother lode! in the 1976 classic film Network