Very massive underground detectors for proton decay searches
  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. 
A. Rubbia