I made a special trip to see the Camberwell submarine the other day. It's not a submarine nor is it an air vent for a Tube line. It's actually ventilation shafts for boilers that heat two council estates nearby.
I discovered this recently after reading this brilliant article about London's hidden portals to the underworld air vents. There's loads of them and once you realise you start seeing them everywhere. This reminded me that when I worked at the Co-op I walked past this thing every week for three years before I worked out that it was a Heatherwick air vent for the Tube.
They're both good, aren't they? Useful and interesting – my favourite combination. I'd love to design one.
There's a more comprehensive list here.