"Grottoes" (grottos?) can be both natural, man-made, or man-modified. Searching for grotto images finds many of these categories. This one struck me as pretty impressive, on the island of Malta. I had to shrink the image a bit; click on it for the full-size image. The second shot is of the notable tourist attraction Blue Grotto of Capri, Italy. There are a lot of pictures on the Web of this place; I couldn't find what I felt was a great one. The glow of the underwater illumination is pretty good; it appears that there is a considerable photographic challenge in...