Sand dunes planted with local vegetation to control their spread in Luis Correa, Piaui state Brazil. 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)

Sand dunes planted with local vegetation to control their spread in Luis Correa, Piaui state Brazil. 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)