Oops. The fares were meant to go up today, but nobody expected fares to be zero — a technical glitch means that Oyster readers aren't working so nobody can touch in or out.
Transport for London is working to fix the problem but until it's sorted, gatelines are open and it's a literal free for all (unless you already have a travelcard). The Guardian quotes a TfL spokesperson
We are experiencing technical issues with our Oyster card readers this morning. We are working extremely hard with our contractors to resolve the issue as quickly as possible. In the meantime, customers can continue to travel across the Tube, bus and rail networks. We apologise for this disruption.
"Apologise"? Don't: it's brilliant. Keep an eye on the TfL Ways to Pay Twitter account for updates.