Sofia (and Bulgaria in general) should be famous for its mineral waters around the world. Instead, they are among its best-kept secrets.
This is a capital where you don’t need to pay for mineral water, you can just refill your bottle. That’s what a lot of locals do, actually. There are a number of mineral water taps around the city but the most popular and most famous is in the very heart of Sofia right next to the Central Public Bathhouse.
Keep in mind that the water is … warm. Most people are pretty surprised by that fact. Not to worry, you just have to be patient enough. People usually fill big bottles enough for a couple of weeks and wait for it to cool off at home. Water is fine for regular use.
The waters should be quite healthy and in the past, they have been used for medical purposes in bathhouses all over the city. Don’t ask me for details about prescriptions but they say the mineral water of Sofia could heal anything but a broken heart.
As for the bathhouses, the central one was turned into a museum of the history of Sofia. The rest, unfortunately, lie in ruins. Therefore, your only option to try swimming in these mineral waters is the outskirts of the city (check the Korali article).
P.S. Tap water is also perfectly safe to drink in Sofia.