Some of the most extreme environments where animals survive are associated with active vents and seeps in the deep sea. In addition to the extreme pressure, low temperatures, and lack of light that characterize the deep sea in general, a variety of other factors that are hostile to most animals prevail in these environments. Hydrothermal vent regions show extremes in temperature, areas of very low oxygen, and the presence of toxic hydrogen sulfide and heavy metals. Hydrocarbon seeps, though much cooler than vents, also have regions of very low oxygen and high hydrogen sulfide, as well as other potentially harmful substances such as crude oil and supersaturated brine. Specially adapted animals not only tolerate these conditions, they often thrive under them. In most cases this tolerance is due to a combination of physiological and behavioral adaptations that allow animals to avoid the extremes of their habitats and yet benefit from the chemoautotrophic production characteristic of these environments.