With data now at hand, not only there are several interesting,
plausible theoretical models within the standard astrophysical
agenda to explain all the CRs detected so far, but there could
indeed be too many. Perhaps yet unexpected degeneracy problems
will appear, even with the forthcoming data of the Pierre Auger
Observatory, a topic which till now has not been a subject of
debate. In our view, Occam's razor imposes that all standard
astrophysical models be eliminated before embarking in the
consequences of explanations involving physics beyond the standard
scenarios. However, should this be the case, the prospect for
encountering a profound scientific revolution are endless. The
puzzle of UHECRs may have something to say about issues as
fundamental as local Lorentz invariance: On the one hand, the
absence of photo-pion production above the GZK-limit would imply
no cosmogenic neutrino flux and possibly undeflected pointing of
the primary back to its source. On the other hand, a significant
correlation of TeV-antineutrinos with directional signals at EeV
energies will validate Special Relativity to an unprecedented
boost factor of 
109,
several orders of
magnitude beyond current limits. Additionally, contrasting the
observed quasi-horizontal neutrino flux with the expected
neutrino flux can help constrain TeV-scale gravity interactions
and improve current bounds on the fundamental Planck scale. An
optimist might even imagine the discovery of microscopic BHs, the
telltale signature of the Universe's unseen dimensions. At the
time of writing, new data is being collected at Pampa Amarilla.
Whatever happens, in T-2 years we shall witness the lift off of a new
era of cosmic ray physics.
Acknowledgments
During the last few years, we have benefitted from discussions with several colleagues and collaborators, among them, Felix Aharonian, John Bahcall, Paula Benaglia, Peter Biermann, Elihu Boldt, Murat Boratav, Yousaf Butt, Analía Cillis, Jorge Combi, Jim Cronin, Tom Dame, Chuck Dermer, Seth Digel, Eva Domingo, María Teresa Dova, Luis Epele, Jonathan Feng, Francesc Ferrer, Haim Goldberg, Francis Halzen, Tim Hamilton, Carlos Hojvat, Dan Hooper, Michael Kachelriess, Mike Loewenstein, Tom Mc Cauley, Gustavo Medina Tanco, Pran Nath, Carlos Nuñez, Tom Paul, Santiago Perez Bergliaffa, Brian Punsly, Olaf Reimer, Steve Reucroft, Andreas Ringwald, Gustavo Romero, Esteban Roulet, Subir Sarkar, Sergio Sciutto, Al Shapere, Paul Sommers, Guenter Sigl, Todor Stanev, John Swain, Tomasz Taylor, Dave Thompson, Peter Tinyakov, Alan Watson, Tom Weiler, and Allan Widom. We would like to thank several of the collegues mentioned above as well as Ray Protheroe for allowing us to use some figures from their papers in this review. We thank Julianna Gianni for inspiration. The work of DFT was performed under the auspices of the U.S. Department of Energy (NNSA) by University of California's LLNL under contract No. W-7405-Eng-48. The work of LAA has been partially supported by the US National Science Foundation (NSF) under grant No. PHY-0140407.