  We study the pressure-induced structural transformation of Ge$_{136}$
clathrate by ab initio molecular dynamics and metadynamics. The system under
pressure first undergoes amorphization followed by an ultrafast
recrystallization to the $\beta$-tin structure on the time scale of 30 ps. The
initial pressure-induced amorphization of clathrate is triggered by high
pressure while the subsequent fast recrystallization to $\beta$-tin is driven
by low temperature. Interestingly, the amorphous intermediate is still
diffusive even at room temperature, in spite of very strong undercooling,
making the ultrafast recrystallization possible. The system provides an
explicit example of structural transformation between two crystalline phases
proceeding via non-crystalline intermediate. Upon fast decompression of the
amorphous structure with incipient crystalline order the recrystallization is
blocked and the system instead proceeds to the tetrahedral LDA amorphous phase.
