In this article, we describe studies, using quantum chemical computations, on possible polycyclization pathways of the farnesyl cation leading to the complex sesquiterpene pentalenene. Two distinct pathways to pentalenene with similar activation barriers are described, each differing from previous mechanistic proposals, and each involving unusual and unexpected intermediates. Direct deprotonation of intermediates on these pathways leads to sesquiterpene byproducts, such as humulene, protoilludene, and asteriscadiene, supporting the notion that a key function of pentalenene synthase, the enzyme that produces pentalenene in Nature, is to regulate the timing and location of proton removal. The implications of the computational results for experimental studies on pentalenene synthase are discussed.