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Pompeii -- the ruined and partially buried Roman city near Naples, Italy -- is one of the most amazing sights one can see in one's lifetime. The city was destroyed during an eruption of the volcano Mount Vesuvius in 79 A.D., but after it was rediscovered in 1738 and excavated in the late 19th century, it became one of the most important archaeological finds (and tourist attractions) of all time.
Now, you can explore it in 3D via Google's Street View, and the results are truly impressive. Of course, the experience is not as good as actually visiting Pompeii in person, but it's definitely the next best thing. Check it out here.