A new class of high modulus polypropylene multifilament fiber and/or yarn is provided. Such a multifilament fiber and/or yarn exhibits an exceptional combination of high strength and toughness with low weight and density. The inventive fibers thus permit replacement of expensive polymeric fibers within certain applications with lower cost alternatives, or replacement of high density components with such low density fibers, without sacrificing strength or durability. Such multifilament fibers are produced through melt-spinning processes and exhibit highly unique microstructures therein, including significant void volumes, interspersed and crossed voids, and nanofilament bridges within such voids. Such microstructural characteristics appear to impart the exceptional properties noted above.