On the southwestern point of Mauritius is a crazy-looking phenomenon. When viewed from the air (either in real life or in Google Earth) it appears to be an underwater waterfall!
Of course, a real underwater waterfall isn’t possible but the actual answer is really quite fascinating. An article on ScienceBlogs.com really digs into it, but the short version is:
What you’re witnessing, that looks like an underwater waterfall, is actually sand from the shores of Mauritius being driven via ocean currents off of that high, coastal shelf, and down into the darker ocean depths off the southern tip of the island.
To see it for yourself, check out the full article on ScienceBlogs or grab this KML file to fly there in Google Earth. [NOTE by Frank: Although Google has 3D bathymetry (underwater terrain), they don't attempt to provide such data so close to shore. Also, I checked a nautical chart, and the area is not as deep as it appears (only 10-20 meters). So, the effect is an optical illusion. But, it does look cool!]