Disclosed herein are various embodiments of systems related to modular earth retaining wall systems. In one embodiment, among others, a precast retaining wall block configured for assembly into a retaining wall includes a front face portion, a web portion extending outwardly from a rear surface of the front face portion, and aligning elements. The web portion includes a vertical center portion and left and right protrusions extending outwardly from the lower sides of the vertical center portion. The left and right protrusions may be substantially triangular. The aligning elements extend downwardly from a bottom surface of the left and right protrusions for locking the precast retaining wall block to the vertical center portion associated with an underlying precast retaining wall block associated with an underlying row.