New observations of burrowing behaviors, nest construction, and sediment-mixing patterns of the western harvester ant Pogonomyrmex occidentalis are described from a neoichnological laboratory study and compared with ichnofossil evidence. Fifty ants burrowed for six weeks in a glass-sided enclosure filled with silty clay simulating a soil with A, C, Ab, and Cb horizons. The galleries, chambers, and mound were mapped and digitally recorded every 12 hours to monitor nest development and changes in architecture. After six weeks the nest was cast with dental plaster to study above- and belowground architectural and surficial nest morphologies. Numerous, intricately interconnected galleries and chambers were constructed, with galleries averaging ~0.9 cm in diameter and chambers varying from ~3 to 10 cm long and up to 1.5 cm high. Burrowing behaviors included pulling, raking, pushing, forcing, cutting, and carrying. Sediment was removed from all horizons and deposited at the surface, resulting in a large mound. Sediment was moved upward and downward within the nest and used to reinforce walls and backfill galleries and chambers. Sediment mixing occurred within and between all horizons as well as from the surface down into the nest. This study demonstrates (1) that ants play a significant role in soil formation, and (2) that these modern ant structures are similar to those found in continental deposits as old as the Late Jurassic, including trace fossils composed of networks of interconnected, variably curved, subhorizontal to subvertical tunnels, some of which are larger in diameter than those described here and have been interpreted as chambers.