  In this manuscript we present a calculation of a physical observable in a
non-perturbative quantum gravitational physical process from covariant Loop
Quantum Gravity. The process regards the transition of a trapped region to an
anti--trapped region, treated as a quantum geometry transition akin to
gravitational tunneling. Figuratively speaking, this is a quantum transition of
a black hole to a white hole. The physical observables are the characteristic
timescales in which the process takes place.
  After an introduction, we begin with two chapters that review, define and
extend main tools relevant to Lorentzian spinfoams and their semiclassical
limit. We then dedicate a chapter to the classical exterior spacetime, which
provides the setup for the problem. In the last two chapters, we arrive at an
explicit, analytically well-defined and finite expression for a transition
amplitude describing this process and use the semiclassical approximation to
estimate the relevant amplitudes for an arbitrary choice of boundary
conditions. We conclude that the transition is predicted to be allowed by LQG,
with a characteristic duration that is linear in the mass, when the process
takes place. The probability for the process to take place is exponentially
suppressed but non-zero, resulting to a long lifetime.
