  Predictive power of the deformed relativistic Hartree-Bogoliubov theory in
continuum (DRHBc) for nuclear mass is examined in the superheavy region $102
\le Z \le 120$. The accuracy to predict the 10 (56) measured (measured and
empirical) masses is $0.635$ ($0.642$) MeV, which is in comparison with $0.515$
($1.360$) MeV by WS4 and $0.910$ ($2.831$) MeV by FRDM. Possible stability
against multi-neutron emission beyond the two-neutron drip line is explored by
the DRHBc theory, which takes into account simultaneously the deformation
effects, the pairing correlations, and the continuum effects. Nuclei stable
against two- and multi-neutron emissions beyond the two-neutron drip line are
predicted in $_{106}$Sg, $_{108}$Hs, $_{110}$Ds, and $_{112}$Cn isotopic
chains, forming a peninsula of stability adjacent to the nuclear mainland. This
stability is mainly due to the deformation which significantly affects the
shell structure around the Fermi surface. The pairing correlations and
continuum influence the stability peninsula in a self-consistent way.
