  The ANITA collaboration has observed two ultra-high-energy upgoing air shower
events that cannot originate from Standard Model neutrinos that have traversed
the Earth. Several beyond-the-standard-model physics scenarios have been
proposed as explanations for these events. In this paper we present some
general arguments making it challenging for new physics to explain the events.
One exceptional class of models that could work is pointed out, in which
metastable dark matter decays to a highly boosted lighter dark matter particle,
that can interact in the Earth to produce the observed events.
