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Tanah Lot, Balinese for "Land in the Sea", is a stunning temple carved into a rock formation on the border of the ocean. Contrusted by the priest Nirartha in the 15th century, it is one of Bali's most sacred sites and it is famous for its marvelous views of the western sunset. Located about 90 minutes north west of Kuta, it's not very difficult to get to and this location is usually part of organized sight seeing tours of the west coast. The rugged coastline here is quite spectacular with many interesting rock formations that interplay with the tides.
If you are in Bali to go sightseeing, then Pura Tanah Lot is a great place to go watch the sunset. Personally, having already travelled Europe, I was not particularly impressed by any of the temples in Bali. I thought they were nice to look at, but for me, the real attraction of Bali is the beach lifestyle and the natural scenery. But it just depends on what experience you are looking to get from your trip. There are also some other cool temples just down the shoreline from Tanah Lot so you can easily spend an afternoon and evening here exploring the coast, checking out the temples, and sampling the local food and shopping.
Check out the Wikipedia Page for more information.