Number of feet per day that a machine known as a roadheader can bore into the earth as it digs out a wine cave. (Soft earth such as volcanic ash allows the machine to move faster). Roadheaders excavate using a rotating core of giant spikes that grind out the earth which is then transported to the rear of the machine and mucked out from the tunnel. At the end of each day, the freshly excavated space is sprayed with a concrete-like mixture to keep the tunnel from collapsing. Several California wine caves exceed 40,000 square feet in size.