But here's my top five facts about the underground.

The system with the most subway stops around the world is New York City in the USA.

It actually has 468 stations.

The world 's busiest metro system in terms of passenger numbers is actually Tokyo 's subway,

It has over eight million passengers per day, that's three billion in a year.

In South Korea, Seoul subway stations are super high tech,

they have 4G and they have wireless broadbands in all their stations and trains

and the seats are actually climate controlled, so they heat up in winter.

It's clean, it's techie, and it 's known as one of the best in the world.

The first ever underground railway was right here in London.

The Metropolitan Railway opened in 1863.

It's now part of the Metropolitan Hammersmith and City and Circle Lines.

The deepest underground system in the world is the Pyongyang Metro System in North Korea,

and it's 110 meters deep.

That's so deep that they also use it as a bomb raid shelter

and it takes three and a half minutes to get to the surface.