  We present multi-epoch global VLBI observations at 2 cm, 3.6 cm and 6 cm of
the compact radio sources in Arp220. We resolve many sources and estimate
sizes, expansion velocities and source classes. We find most source properties
are consistent with them being either radio supernovae (SNe) or supernova
remnants (SNRs). We extend the luminosity-diameter relation for SNRs to very
small sources and argue this supports models where shell magnetic fields are
internally amplified. We also detect one probable SN/SNR transition object
candidate and three highly variable sources with possible superluminal motion
(approximately 4c) of jet-like features near rapidly varying almost stationary
components. These enigmatic sources, which show similarities to the recently
discovered superluminal source in M82, might be associated with an AGN or a new
radio source class (e.g. intermediate-mass black holes or micro-blazars).
