At home I have an Oyster card that I no longer use on top of my fridge which, I think, has £1 left on it.
I am not alone in leaving small amounts of cash on unused Oyster cards.
There are 17m cards out there that have not been used for at least 12 months with a combined balance of an eye-watering £55m.
The Liberal Democrats, who obtained the figures using a Freedom of Information request, say it is not easy to get the money back from unused cards.
Caroline Pidgeon, leader of the Liberal Democrat London Assembly Group, said: "Transport for London (TfL) is now sitting on a cash pile of £55m which should be returned to passengers.
"They [TfL] claim people can easily reclaim their money from dormant Oyster cards but the evidence suggests otherwise. If it really was simple for passengers to reclaim their money, why has the amount left on dormant Oyster cards doubled in the last two years?"
She accused TfL of being "shy" about publicising how people can get their own money back.
Shashi Verma, TfL's director of customer experience, said: ''Pay as you go Oyster cards do not expire and customers are able to return their cards to a Tube ticket office at any time for a refund of the remaining balance and card deposit.
"Registered Oyster card users are also able to claim balance refunds without presenting their card, for example if a card is lost or stolen."
In your experience, is it easy to get the money back?