Stockholm sits where a huge freshwater lake (Lake Mälaren) meets the salty Baltic Sea. The water around the city splits into hundreds of channels and bays, so the land breaks up into islands. The city grew up over the years by hopping from island to island.
The oldest part is called Gamla Stan, which means 'Old Town' in Swedish. It is built on a tiny island in the middle of the water. The streets are narrow and cobbled, the houses are painted yellow, orange and red, and the whole place looks almost exactly like it did 400 years ago.
Just outside Stockholm, there is something even more amazing: the archipelago. That's a fancy word for a group of lots of islands close together. The Stockholm archipelago has around 30,000 islands. Most are tiny - some just a single rock big enough for a few trees. People take boats out to them in summer to swim and have picnics.
Because there is so much water, Stockholm is one of the cleanest big cities in the world. You can fish in the middle of the city, and the water is clean enough to swim in.