The Zelos is a perfect lightweight men's hydration pack for all summer mountain sports. Initially designed for trail running, like all Nathan packs, the Zelos features a 3-way propulsion strap to keep the base of the pack in place without the need for a hip belt. This is also a great feature for any kind of technical hiking or climbing, especially where a climbing harness might usually interfere with a hip belt. The 2 Liter bladder is supplemented by an 18-oz. flask on the front strap for additional water or nutrient supplements. The Zelos is also upgraded with 7 liters of storage for your gear and an external bungee strap to secure spare clothes.