  The Cosmic Ray Extremely Distributed Observatory (CREDO) is a newly formed,
global collaboration dedicated to observing and studying cosmic rays (CR) and
cosmic ray ensembles (CRE): groups of a minimum of two CR with a common primary
interaction vertex or the same parent particle. The CREDO program embraces
testing known CR and CRE scenarios, and preparing to observe unexpected
physics, it is also suitable for multi-messenger and multi-mission
applications. Perfectly matched to CREDO capabilities, CRE could be formed both
within classical models (e.g. as products of photon-photon interactions), and
exotic scenarios (e.g. as results of decay of Super Heavy Dark Matter
particles). Their fronts might be significantly extended in space and time, and
they might include cosmic rays of energies spanning the whole cosmic ray energy
spectrum, with a footprint composed of at least two extensive air showers with
correlated arrival directions and arrival times. Since CRE are mostly expected
to be spread over large areas and, because of the expected wide energy range of
the contributing particles, CRE detection might only be feasible when using
available cosmic ray infrastructure collectively, i.e. as a globally extended
network of detectors. Thus, with this review article, the CREDO Collaboration
invites the astroparticle physics community to actively join or to contribute
to the research dedicated to CRE, and in particular to share any cosmic ray
data useful for the specific CRE detection strategies.
