  Possible bound nuclei beyond the two-neutron drip line in the $50\leqslant Z
\leqslant 70$ region are investigated by using the deformed relativistic
Hartree-Bogoliubov theory in continuum with density functional PC-PK1. Bound
nuclei beyond the drip lines of $_{56}$Ba, $_{58}$Ce, $_{62}$Sm, $_{64}$Gd and
$_{66}$Dy are predicted, forming peninsulas of stability in nuclear landscape.
Near these peninsulas, several multi-neutron emitters are predicted. The
underlying mechanism of the peninsulas of stability is investigated by studying
the total energy, Fermi surface, quadrupole deformation and the single-neutron
spectrum in the canonical basis. It is found that the deformation effect is
crucial for forming the peninsulas of stability, and pairing correlations are
also essential in specific cases. The dependence on the deformation evolution
is also discussed. The decay rates of multi-neutron radioactivity in Ba and Sm
isotopic chains are estimated by using the direct decay model.
