  LHC searches for new physics focus on combinations of hard physics objects.
In this work we propose a qualitatively different soft signal for new physics
at the LHC - the "anomalous underlying event". Every hard LHC event will be
accompanied by a soft underlying event due to QCD and pile-up effects. Though
it is often used for QCD and monte carlo studies, here we propose the
incorporation of an underlying event analysis in some searches for new physics.
An excess of anomalous underlying events may be a smoking-gun signal for
particular new physics scenarios such as "quirks" or "hidden valleys" in which
large amounts of energy may be emitted by a large multiplicity of soft
particles. We discuss possible search strategies for such soft diffuse signals
in the tracking system and calorimetry of the LHC experiments. We present a
detailed study of the calorimetric signal in a concrete example, a simple quirk
model motivated by folded supersymmetry. In these models the production and
radiative decay of highly excited quirk bound states leads to an "antenna
pattern" of soft unclustered energy. Using a dedicated simulation of a toy
detector and a "CMB-like" multipole analysis we compare the signal to the
expected backgrounds.
