Caliche is a poor-mans concrete and is naturally formed. It is a term for calcium carbonate (CaCO3) that forms in sediments or in voids and crevices within bedrock just below the surface in semiarid regions, as a result of soil-forming processes or ground-water evaporation.
It is usually a material left behind by evaporation of ground water.
The layers are a few inches to several 4 or more feet thick, and acts as a percolation barrier of soil from precipitation or artificial watering.
The majority of the soil in the Las Vegas area is underlaid with a lense of caliche which must be exposed and/or penetrated to facilitate runoff and for planting or large shrubs and trees.
There is a chance that this material was man-placed in a tunnel for whatever purpose.