Weighing in at an ultralight 7. 6 ounces, the Mammut MTR 201 Hydration Pack qualifies as the world's lightest trail running backpack. This hydration pack features breathable, fast drying VenTech material to combat the nasty, sweat logged back panel often found after a long distance day on the trail. Packed with a surprising number of pockets for its tiny size, this hydration pack has a large main zippered compartment as well as two side mesh pockets for stuffing snacks or extra water bottles. A three compartment pocket on the left shoulder strap holds tiny items and accessories within easy reach when you're running or hiking. In addition, the right shoulder strap pocket holds an extra water bottle or light snack for on trail sustenance. As if that weren't enough, this pack will hold a small hydration reservoir sold separately, and it's equipped with an elastic bungee cord for securing lightweight jackets or other accessories.