  Despite more than a quarter century of research, the nature of the
second-order phase transition in the heavy-fermion metal URu$_2$Si$_2$ remains
enigmatic. The key question is which symmetry is being broken below this
"hidden order" transition. We review the recent progress on this issue,
particularly focusing on the thermodynamic evidence from very sensitive
micro-cantilever magnetic torque measurements that the fourfold rotational
symmetry of the underlying tetragonal crystal is broken. The angle dependence
of the torque under in-plane field rotation exhibits the twofold oscillation
term, which sets in just below the transition temperature. This observation
restricts the symmetry of the hidden order parameter to the $E^{+}$- or
$E^{-}$-type, depending on whether the time reversal symmetry is preserved or
not.
