Not entirely unlike Washington, D.C., the site of what would become Buckingham Palace was once a marsh, fed by the River Tyburn. Now centuries and plenty of building projects later, the river still flows—beneath parts of the official London residence of the British royal family. The palace above it today hosts 775 rooms, a secret swimming pool, and the largest private garden in London. 

There are plenty more superlatives to be found around Buckingham Palace, from what might be the world’s oldest hat store to what is probably Europe’s largest dog library. Here are some of our favorite unusual sights once you get your fill of the towering hats of the King’s Guard.