March 24 is the 115th anniversary of the groundbreaking of what would become the New York City subway system.

Here are a few facts about the New York Subway you might not have known.

First: Despite its name, 40 percent of the subway system is not underground.

Second: At one point there were three subway systems operating in New York City.

The privately owned Brooklyn-Manhattan Transit company (known as BMT) and Interborough Rapid Transit Company (known as IRT), and the city-owned Independent Subway System. The city bought the privately owned companies in 1940.

Third: The total length of the system is 232 miles, making it the fourth-longest metro system in the world.