The Explorer launched in 1953, the same year as the Submariner, and took its name from Rolex's run of supplied watches on Himalayan expeditions through the early 1950s. The Hillary and Norgay Everest ascent that May supplied the marketing peg, though the dial layout had already been settled on the 6150. The line is defined by three things: a 36mm Oyster case (until the 214270 stretched it to 39mm in 2010), a 3-6-9 Arabic-numeral dial, and Mercedes hands. No date, no rotating bezel, no complications of any kind.
The Explorer II arrived in 1971 with a 24-hour hand aimed at cavers and polar workers who lose track of day and night. It runs across five references (1655, 16550, 16570, 216570, 226570) on its own movement family and is documented separately. This page covers Explorer I.
Hillary and Norgay's 1953 Everest expedition — the event that launched the Explorer name