Formed by explosive eruptions thousands of years ago, this stunning crater-lake is fed by underground streams from Kilimanjaro. During the World War 1, the allied forces marched around the northern edge to battle it out with the Kaiser’s force. It’s 4.5 square kilometres and 94 metres deep with the invisible line separating Kenya and Tanzania and you can enjoy a swim in it at your own risk for even though the locals say that Grogan’s crocodiles have all been killed, there might be one just lurking about.