  We observed a new isomeric gamma transition at 168 keV in $^{36}$Mg, with a
half-life of T$_{1/2}$=[130-500]$(\pm40)(^{+800}_{-20})_{sys}$ ns. We propose
that the observed transition de-excites a new 0$^+$ isomeric state and
populates the previously known first 2$^+$ state. The existence of this isomer
is consistent with the predictions of the large-scale shell model calculations
of $^{36}$Mg using the sdpf-u-mix interaction. The observed excitation energy
of the second 0$^+$ state is caused by the small energy separation between two
prolate-deformed configurations where the intruder configuration corresponds to
two neutron excitations from the {\it sd} to the {\it pf} shell. Within this
interpretation, $^{36}$Mg becomes the crossing point between nuclei in which
ground state deformed/superdeformed configurations are caused by the dominance
of N=20 intruders ($^{32,34}$Mg) and nuclei where deformed configurations are
associated with N=28 intruders ($^{38}$Mg and beyond). We found the lack of
three-body monopole corrections in other effective interactions results in a
predominance of N=20 intruder configurations past $^{38}$Mg incompatible with
our observation. We conclude that $^{36}$Mg bridges the N=20 and N=28 islands
of inversion, forming the so-called Big Island of Deformation.
