A few facts. Mandolay Bay is in Clark County, not the City of Las Vegas. Their building department is one of the best in the world...only continuous, ie. every inch, of test borings could uncover unique underground conditions. This is not financial practical. Sample borings are taken and building takes place according to the results. This is a desert environment and the soil in the Vegas valley is a result of erosion from the nearby mountains during pre-desert conditions. As a result, there are numerous deep but narrow deposits of a clay-like material called caliche. When thoroughly wet, it acts like clay but when dry is harder than cement. Easy to miss these pockets when boring. So, don't condemn the building department. No I don't work for the County.