Ah, the Tube, or, as some call it, the Underground. It's a rapid transit train system (subway, really) built under London, and it's amazing. It services the main London area, though there are a few lines that travel out a little bit farther.
We loved that we could get pretty much wherever we wanted to be within a reasonable amount of time, just by taking the Tube. Now, don't get me wrong--there were times where it was easier, shorter, or faster to walk on the surface between stops, or where line closures made life a little difficult, but the benefits outweighed the costs.
The London Underground just celebrated its 150th anniversary a couple of years ago. Obviously there were some huge changes--lines being added; the Oyster card being added in 2003 (a contactless ticketing system that makes life super easy), lifts and escalators, etc.
Compared to other similar transport systems in mainland Europe (the Paris Metro, for example), the London Underground is clean, well-lit, and (fairly) reliable. There's even multiple apps in case you need to know how to get from A to B.
I might be a little obsessed with things with the Tube map on them. I have some mugs, a magnet, and I'd happily have a map stenciled on my wall, but I think my poor husband might object. Though saying, "This is the Piccadilly line train to Cockfosters" will never not be funny.
*note* The Tube map doesn't belong to me, obviously.
Time for V next!