A large cloud chamber containing 12 Pb plates (1 cm thick) and internal Geiger counters has been operated for 5 000 h at 65 m w.e. underground. 28 penetrating showers produced inside the chamber by high energy μ-mesons have been observed. These showers have been analysed with the following results: 1) As far as the relative number of secondary π+-, π0 and protons is concerned, the penetrating showers observed underground do not differ significantly from those obtained at sea-level and mountain altitudes. The observed mean free path for star production and nuclear scattering by the ionizing secondaries (the outgoing μ-meson being excluded) is 233 ± 52 g/cm−2 Pb. This corresponds to a geometrical mfp, taking into account the finite plate thickness. 2) The angle of deviation of the μ-meson in the interaction is shown to be very small, less than 2° in most cases. This is about one order of magnitude less than the estimated ratioq/E of energy transfer to primary energy, in agreement with the interpretation of the events in terms of inelastic photo-nuclear interactions. 3) The mean number of ionizing penetrating secondaries (without the outgoing μ-meson) is 3.7. The mean energy of the observed events has been estimated from the electron cascades produced by disintegration of π-mesons: 16 GeV. 4) The observed cross-section of (0.43±0.08)·10−30 cm2/nucleon for production of penetrating showers of more than 12 GeV by μ-mesons at 65 m w.e. underground is in good agreement with the predictions of a refined Williams-Weizsacker theory and an assumed photo-nuclear cross-section of 10−23 cm2/nucleon at those high energies. 5) There is some evidence that the secondaries are not emitted isotropically in the center-of-momentum system of the virtual photon and the target nucleon. An anisotropic distribution of the type (1+cosΦ) dΘ) in the CM system would agree with the observed angular distribution in the laboratory. In the same experiment, we have observed 12 cases of parallel non-interacting penetrating particles, presumably μ-mesons from extensive air showers. 2 pictures show as much as 9 and 11 parallel μ-mesons in the illuminated region of the cloudchamber, which leads to a density of about 55 particles per m2 at 65 m w.e. and is estimated to arise from air-showers of about 3·1018 eV. This energy estimate would agree with the observed frequency.