...and then quickly disappears, after naive, but diligent cleaners remove the graffiti.
'The piece, If You Don't Mask, You Don't Get, was painted inside a Circle Line service carriage.
As usual when it comes to Banksy, this seems to have been part of the plan all along, and part of the reason that the art itself is worth such an enormous amount of money.
Manufacturing scarcity is a wonderful mechanism for increasing the value of something, along with a great story of course. Both of which, Banksy does very well.