Reward for The Ankh (Nightmare) achievement The Key to Life, part of the Master Planner achievement set. The outfit comes in three parts: Spelunking Helmet, Spelunking Glasses and Spelunking Uniform (which includes the backpack.)
Note: When the outfit was originally a single piece Multislot item, the glasses were white as shown in the male screenshot. The eye wear was replaced with the darker sunglasses, as seen in the female screenshot, when the Multislot item was converted in to a three piece set.
Thank you to Scrula and Deathstalker for submitting the screenshots.