  When matter is cooled from high temperatures, collective instabilities
develop amongst its constituent particles that lead to new kinds of order. An
anomaly in the specific heat is a classic signature of this phenomenon. Usually
the associated order is easily identified, but sometimes its nature remains
elusive. The heavy fermion metal $URu_2Si_2$ is one such example, where the
order responsible for the sharp specific heat anomaly at $T_0=17 K$ has
remained unidentified despite more than seventeen years of effort. In
$URu_{2}Si_{2}$, the coexistence of large electron-electron repulsion and
antiferromagnetic fluctuations in $URu_2Si_2$ leads to an almost incompressible
heavy electron fluid, where anisotropically paired quasiparticle states are
energetically favored. In this paper we use these insights to develop a
detailed proposal for the hidden order in $URu_2Si_2$. We show that
incommensurate orbital antiferromagnetism, associated with circulating currents
between the uranium ions, can account for the local fields and entropy loss
observed at the $17 K$ transition; furthermore we make detailed predictions for
neutron scattering measurements.
