It is important to mention that these ads are a subset of the under-ground housing market. The landlords that are posting ads must be ed-ucated and resourced enough to post these ads. Furthermore, many of the ads post photos of the units, advertising its relatively good quality. This also implies that the landlords must feel relatively secure about their tenure and may have already received a Certificate of Use. There-fore, this is a study of a segment of a sub-market, most likely the upper-end quality of subterranean housing.