The volcano has a well formed summit crater. A small crater lake is found within the Aguas Calientes summit crater, making it one of the highest lakes in the world at an altitude of . The lake has a surface area of and is tinged red from a population of microorganisms. The lake water is acidic and is frequently influenced by activity from Lascar, with winds carrying sulfuric acid and water vapour clouds to the crater lake. Precipitation was measured at with most precipitation falling during the winter months, and strong UV irradiation. The lake has a bacterial ecosystem that is primarily reliant on photosynthesis but may also be influenced by hydrothermal activity from Aguas Calientes volcano. Research has found only limited numbers of bacterial taxa and a large number of DNA sequences that can't be identified as known phylogenetic groups. The lake has been studied as a possible terrestrial analogy to Mars.