  Combination of neutron scattering and thermal expansion measurements under
pressure shows that the so-called hidden order phase of URu2Si2 reenters in
magnetic field when antiferromagnetism (AF) collapses at H_AF (T). Macroscopic
pressure studies of the HO-AF boundaries were realized at different pressures
via thermal expansion measurements under magnetic field using a strain gauge.
Microscopic proof at a given pressure is the reappearance of the resonance at
Q_0=(1,0,0) under field which is correlated with the collapse of the AF Bragg
reflections at Q_0.
