Due to its location near the Atacama desert, the mountain has very dry conditions with snow only remaining on the peak during winter. Despite the generally dry conditions, there is a permanent crater lake about 100 metres (300 ft) in diameter at an elevation of 6,390 m (20,960 ft) on the eastern side of Ojos del Salado. This is most likely the highest lake of any kind in the world.
The ascent of Ojos del Salado is mostly a hike except for the final section to the summit which is a difficult scramble that may require ropes. The first ascent was made in 1937 by Jan Alfred Szczepański and Justyn Wojsznis, members of a Polish expedition in the Andes.
Its name comes from the enormous deposits of salt that, in the form of lagoons or “eyes”, appear in its glaciers.
There is no doubt that Ojos del Salado is a recently active volcano, but the question of whether it should be considered currently (or "historically") active is arguable. According to the Smithsonian Institution's Global Volcanism Program, the most recent known eruption was around 1300 years ago, with large error bars. However, there is also some evidence for a minor ash emission in 1993, which would definitely qualify the volcano as historically active.
The presence of fumaroles high on the mountain and recent-looking lava flows, albeit of uncertain age, also argues in favor of a categorization as "active." By these definitions Ojos del Salado is the highest historically active volcano on earth.
If the older date is accepted, the title of "highest historically active volcano" might reside instead with the somewhat lower Llullaillaco volcano, which certainly has erupted in historic times (most recently in 1877) and should still be considered active. Definitions of "active" being themselves controversial and somewhat arbitrary, the point is mainly of trivial intere.