Antonio Souza de Santos walks on sand dunes which now cover his and 26 other homes in the town of Ilha Grande in the northeastern Brazilian state of Piaui. While the magnificent dunes of Brazil's northeast are a natural occurance, in some places a perfect storm of factors, including unchecked human impact on the land, has lead to what amounts to a tropical version of a glacier. Powered by the wind, the dunes advance a few meters per year, overtaking anything in it's path. In the town of Ilha Grande, an entire neighborhood is underneath the sand. Experts say the sand may even block the Parnaiba River from reaching the ocean, which would have far-reaching environmental and social repercussions. (Australfoto/Douglas Engle)

Antonio Souza de Santos walks on sand dunes which now cover his and 26 other homes in the town of Ilha Grande in the northeastern Brazilian state of Piaui. While the magnificent dunes of Brazil's northeast are a natural occurance, in some places a perfect storm of factors, including unchecked human impact on the land, has lead to what amounts to a tropical version of a glacier. Powered by the wind, the dunes advance a few meters per year, overtaking anything in it's path. In the town of Ilha Grande, an entire neighborhood is underneath the sand. Experts say the sand may even block the Parnaiba River from reaching the ocean, which would have far-reaching environmental and social repercussions. (Australfoto/Douglas Engle)