  The events of multiple neutron production under 2000g/cm$^2$ thick rock
absorber were studied at the Tien~Shan mountain cosmic ray station, at the
altitude of 3340m above the sea level. From comparison of the experimental and
Geant4 simulated neutron multiplicity spectra it follows that the great bulk of
these events can be explained by interaction of cosmic ray muons with internal
material of the neutron detector. In synchronous operation of the underground
neutron monitor with the Tien~Shan shower detector system it was found that the
characteristics of the muonic component of extensive air showers which is
seemingly responsible for generation of the neutron events underground do
change noticeably within the energy range of the knee of primary cosmic ray
spectrum. Some peculiar shower events were detected when the neutron signal
reveals itself only $\sim$(100--1000)\,$\mu$s after the passage of the shower
particles front which probably means an existence of corresponding delay of the
muon flux in such events.
