Marunouchi Line
The Tokyo Metro Marunouchi Line (丸ノ内線) literally means "Line within the Circle", and naive Tokyoites without any sense of history might think this is because most of it passes largely within the circular Yamanote Line. They would of course be utterly wrong. The "circle" referred to here is one of the concentric circles of the old Edo Castle fortifications and the name applies to the business district located between Tokyo Station and the Imperial Palace.
Tokyo's second-oldest underground line, it was the first postwar line to be constructed and runs in a sweeping yet pointy semicircle from Ogikubo on the Chuo Line via Shinjuku via the Ginza area to Tokyo Station in the aforementioned Marunouchi district, before swinging back out to Ikebukuro where it comes to an abrupt stop, which is quite understandable as there is not much beyond Ikebukuro worth going to.