One thing I loved loved loved about Rome was all of the fountains! Some call Rome the City of Fountains. Supposedly there are somewhere around 300 fountains within the Rome City Walls, the most famous of which is the Trevi Fountain. However, it's actually kind of hard to enjoy the Trevi Fountain because of the hordes of camera wielding tourists around it
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I really enjoyed exploring some of the lesser known fountains of the city. Of course, the most amazing thing is that all of the fountains supply perfectly clean drinking water! Since Ancient Roman times, Rome has been gifted with widely available drinking water. The extensive system of Roman aqueducts still functions today! Talk about engineering! Here are some of my other favorite fountains...
Fontana dell'Acqua Paola (the square in front of this fountain has a magical panoramic view of Rome as well)
Fontana del Tritone
For a nerdy, but awesome website cataloging Roman fountains