This paper reports recent progress of ultrasonic measurements on URu$_2$Si$_2$, including ultrasonic measurements under hydrostatic pressure, in pulsed-magnetic fields, and the effect of Rh-substitution. The observed changes of the elastic responses shed light on the orthorhombic-lattice instability with $Gamma_3$-symmetry existing within the hidden order and the hybridized 5$f$-electron states of URu$_2$Si$_2$.
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