Piazza Mattei, named after an important family of the Middle Ages, hosts the Tortoise Fountain. One of the most beautiful fountains in Rome, it was designed by Giacomo Della Porta at the end of the 16th century. The tortoises were added in 1658 by Bernini.
You found the Fountain of the Turtles! Congratulations! It must mean you've been wandering Rome, as it's not on the well beaten path. It was designed by Giacomo della Porta (but Bernini added the turtles). Della Porta was an important sculptor and architect who finished Michelangelo's Dome of St. Peter's.
If you go to see the three famous Carravaggio paintings on the life of St. Matthew at the church San Luigi dei Francesi (St. Louis of the French), you'll be in another della Porta building - well, the facade anyway.
This fountain was completed in 1585 and the water comes from the Acqua Vergine, said to contain the best water of all the aquaducts. It is so charming, it always makes me smile when I see it.
UPDATE, MARCH 5, 2006: VT member Baronedivandastad had been torturing me for months....he knew a secret about the fountain, but he wouldn't tell me until we met for dinner in Rome. We finally met and with great fanfare, he disclosed the secret at a wonderful VT dinner in Testaccio. I think I'll keep the tradition. No telling of the secret unless we have dinner together in Rome!
If you want to look for the Fontana delle Tartarughe, see my directions below, but PROMISE me, you'll wander after you find it. The neighborhood is great for wandering. You're close to the Jewish Ghetto, a perfect excuse to stop for some artichokes fried in the Roman Jewish tradition. Or kosher pizza - yes, really.
The fountain is in Piazza Mattei. Find "Largo Argentina," the excavated Roman site, which is is on the south side of Corso Vittorio Emanuele II (just to the west of Piazza Venezia ). Piazza Mattei is just south of that.
Another lesser known fountain is the "Fontana delle Tartarughe" in the Piazza Mattei.
This lovely Renaissance sculpture is one of Rome's more secret fountains showing youths helping tortoises into a basin.
Beautiful fountain featuring turtles which were added at a later date and which are thought to be by Bernini.