CRATER Lake is located in a caldera on Mt Mazama, a volcanic centre in the Oregon Cascades which has been active for more than 400,000 years1. The 594-m-deep lake is a consequence of a climactic eruption which occurred 6,845 ±50 years ago; however, caldera volcanism took place as recently as 4,000 years ago1. It has been suggested that some of the physical and chemical features of the lake result from hydrothermal inputs2-7. Here we present submersible observations of the bottom of Crater Lake, which reveal communities of bacteria that are usually associated with anomalously warm, saline waters. The bacteria seem to derive energy from the oxidation of ferrous iron to fuel their metabolism. We propose that the mats are indicators of diffuse hydrothermal venting into the deep lake.
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