In 1909 the European seismologist Mohorovičić “discovered” the layering and the crust of the earth by tracking the arrival times of earthquake waves across a network of seismic monitoring stations. Many geologists think that the crust of the earth contains its most ancient rocks. The thickness of the crust bears on problems of earthquake hazards and in the search for geothermal energy and critical mineral resources. A variety of seismological analyses can determine crustal thickness. These methods are largely based on recordings of earthquakes from traditional seismic monitoring sensors, as they were in Mohorovičić’s time. Now, a collaboration between two companies- Terēan and Sintela- and the Univ. of Nevada has been able to estimate crustal thickness in Western Nevada using an optical fiber, part of a telecommunications cable stretching 56 km east from Reno along a major freeway. The seismic signals they used are referred to as microtremor, vibrations in the ground emanating from traffic and industrial activities. The Terēan data analysis identifies the speed of microtremor signals with wavelengths as long as 25 km traveling along the fiber. The speeds noted suggest a crustal thickness of 35 km in the area, similar to previous workers.