Io is extremely active geologically with many sulphur spewing volcanoes. Early explorers of the moon found hotspots where black sulphur boiled to the surface but were too small to be considered volcanoes. While the surrounding terrain could be as cold as -145°C, a hotspot could reach 200°C. The surrounding cold would quickly form a thin layer of solid sulphur on the surface of a hotspot which would be covered with the usual yellow sulphur dust. These were still very dangerous as the layer would not support the weight of a crawler. Hence, crawlers were equipped with infrared sensors so drivers could avoid them.