It is called the because of it's shape. If you look at the Paris Underground it is square but the London Underground has tube shaped tunnels and stations. Even the trains are tube shaped, apart from the lines which go overground for quite a way like the District and Circle line.
The Northern Line, which is part of the London Underground metro line, also known as The Tube, transports around 252,310,000 passengers a year. That makes it the second busiest line in the London Underground.