In 1930, the Shubnikov–de Haas effect and the de Haas–van Alphen effect were discovered in bismuth (a metal). In the same year, Landau published his theory of Landau quantization that explains the experimental observations. By the 1970s, these effects, known as quantum oscillations, were established as the standard quantum characteristics, as well as diagnostic tools, of almost all metals, but not insulators. The same underlying process of Landau quantization further gave rise to the discoveries of integer and fractional quantum Hall effects in the 1980s in the high-mobility metallic two-dimensional electron gas (2DEG), which are the foundation of the nowaday field of topological quantum matter. In this talk, I will describe our recent findings of Landau quantization in the insulator states of WTe₂, a phenomenon that came as a complete surprise. Despite being a strong insulator, the observed resistance of monolayer WTe₂ displays large quantum oscillations with many periods onsetting at a very low magnetic field, and displays discrete peaks, mimicking those observed in the high-mobility 2DEG. I will discuss both the experimental observations and their intriguing relation to the formation of ground-state excitons and possible charge-neutral fermions. The observations call for an understanding of a new type of quantum insulators that host hidden neutral excitations and phases.