This dome leads to a tunnel that passes under the Thames. Opened in 1902 and linking East London to Greenwich, its purpose was to replace the ferry service for the shipyards' workers. If you look carefully in the background of this photo, to the right you'll see the other end across the Thames. (@food_and_wonder , please correct me if I get some of the details wrong!)
Walking around the Royal Navy Colleges, I couldn't stop thinking how much it reminded me of Cambridge. Except with the Thames, the Shard and the rest of the London skyline in the background. Thanks for the tour, @food_and_wonder & @oscar_asterix !