Rolex's dedicated adventure watch, the Explorer, traces its origins back to mountain climbing in 1953. In 1971, however, another watch joined the collection with the name Explorer II and a quite different set of features, looks, size and purpose. The original Explorer II was intended for plumbing the depths of caves. A 24-hour hand and bezel were meant to keep the likes of spelunkers or arctic explorers oriented when it might be easy to lose track of whether it's day or night. Unlike on the Rolex GMT-Master, the bezel was fixed (non-rotating) and the 24-hour hand couldn't be set separately from the main time. There have been different Explorer II iterations to date, but the Rolex Explorer II that is well known today was more or less established in 1985. The differences between the models have sometimes been subtle and other times significant, so here's a breakdown of how this iconic watch has evolved over the decades.