  Grand Unification of the strong, weak and electromagnetic interactions into a
single unified gauge group is an extremely appealing idea which has been
vigorously pursued theoretically and experimentally for many years. The
detection of proton or bound-neutron decays would represent its most direct
experimental evidence. In this context, we studied the physics potentialities
of very large underground Liquid Argon Time Projection Chambers (LAr TPC). We
carried out a detailed simulation of signal efficiency and background sources,
including atmospheric neutrinos and cosmogenic backgrounds. We point out that a
liquid Argon TPC, offering good granularity and energy resolution, low particle
detection threshold, and excellent background discrimination, should yield very
good signal over background ratios in many possible decay modes, allowing to
reach partial lifetime sensitivities in the range of $10^{34}-10^{35}$ years
with exposures up to 1000 kton$\times$year, often in quasi-background-free
conditions optimal for discoveries at the few events level, corresponding to
atmospheric neutrino background rejections of the order of $10^5$. Multi-prong
decay modes like e.g. $p\to \mu^- \pi^+ K^+$ or $p\to e^+\pi^+\pi^-$ and
channels involving kaons like e.g. $p\to K^+\bar\nu$, $p\to e^+K^0$ and $p\to
\mu^+K^0$ are particularly suitable, since liquid Argon imaging (...)
