The decoherence curve of extensive showers has been studied at a depth of 60 m water equivalent below ground out to a distance of 320 m. The relative frequencies of coincidences in various combinations of counter sets are found to be consistent with the assumption that the frequency of showers containing N particles is proportional to N^-γ with γ = 3.2 ± 0.2. By comparison with observations at other depths it is concluded that the particles in extensive showers are absorbed less rapidly in the earth than the normal particles of the vertical component. The observed showers can be readily understood in terms of the small fraction of penetrating particles (presumed μ-mesons) that is known to exist in extensive showers observed at sea level. At counter separations less than 3 m the observed coincidences are much too frequent to be attributed to extensive showers, indicating a significant contribution from locally produced penetrating particles. The origin of these secondary particles is discussed.