Mexico City's Metro Line 3 is almost the longest line in the system. For international visitors, it's arguably the most important. Running mostly north-south, it bisects much of the city, and arrives at many of the most important places visitors want to see.
Like all of the Metro, the line also provides a mental map for everyone in the city. It's a continual reference point as to the geography and layout of the entire metropolitan area.
Running from Indios Verdes in the north to the Universidad Station in the south, the entire route is just a bit more than 21 kilometers long. With 21 stations in all, the line has been extended five times.
There's a story, or at least something to learn, in every station. For tips on using the Mexico City Metro, including Metro Line 3, see the Getting Around section in the Essential Guide to Mexico City.