Think of it as New York’s biggest sock drawer, except that instead of nickels, dimes and quarters, what is squirreled away in its dark recesses are millions of lapsed yellow-and-blue MetroCards with digital loose change still dangling from their magnetic strips.

In the decade ending in 2010, nearly $500 million worth of unspent balances on expired bus and subway MetroCards accumulated, and that money can no longer be redeemed.

Cards that are bought, never used but still valid are counted for bookkeeping purposes as a liability, because they might eventually be used. Outdated cards with pending balances become an asset after they expire, about two years from the date of sale. The balances are listed as revenue under the category of “fare media liability.”

Tens of millions of dollars a year may not seem like much out of $4 billion in annual MetroCard revenue for New York City Transit, but there is no stream of cash that the agency scoffs at.