  Massive underground detectors can be considered as sort of observatories for
rare physics phenomena like astrophysical neutrino detection and nucleon decay
searches. We briefly overview the past tracking calorimeters developed for
nucleon decay searches. We then discuss the two technologies which are
discussed today for potential future applications: Water Cerenkov ring imaging
(WC) and liquid Argon Time Projection Chamber (LAr TPC). We present a
conceptual design for a 100 kton liquid Argon TPC. We illustrate the physics
performance of Water Cerenkov and liquid Argon TPC detectors for the $p\to
e^+\pi^0$ and $p\to \nu K^+$ proton decay searches. We briefly compare the
physics reach of the two techniques. We conclude by stressing the
complementarity of the two approaches, noting however that, given the
foreseeable timescale for these next generation experiments, the new
challenging technique of the LAr TPC might offer more discovery potentials.
