Load-bearing structures in underground mines are often required to carry additional loads while mining operations are in progress. As the structures adjust to this new weight small-scale displacements take place that result in the release of seismic and acoustic energy called "rock noise." This energy can be detected using special geophysical equipment. Assume that in a particular mine the average number of rock noises recorded during normal activity is three per hour. Would you consider it unusual if more than 10 were detected in a two-hour period? Explain, based on the probability involved. (Based on "A multichannel rock noise monitoring system," T. Gowd and M. S. Rao, Journal of Mines, Metals and Fuels, September, 1981, pp. 288-290.)