Detailed mapping of bedrock topography on Bahia Honda and Big Pine Keys has revealed a buried karst topography not previously documented in the lower Florida Keys. This topography, developed during lowered sea level of the Pleistocene, strongly controls Holocene sediment thickness and present biotic distribution. Circular to oval sinkholes, which are up to 75 m or more in diameter and over 4 m deep, are usually completely filled with peat and carbonate sediment. Sinkholes are well developed on both the Miami Limestone (oölitic facies) and the Key Largo Limestone (both late Pleistocene in age). Thick sediment in buried sinkholes in more than a few inches of water favors the growth of thick patches of turtle grass (Thalassia testudinum). Shallower water and supratidally located sinkholes (that is, those partly or wholly surrounded by subaerially exposed bedrock) are generally marked by thick growths of either red or black mangroves (Rhizophora mangle and Avicennia nitida). These distinct, nearly circular vegetation patterns are extremely abundant in the study area, as shown by aerial photographs which suggest that Bahia Honda and Big Pine Keys are “riddled” with sinkholes.