  M87 is one of the nearest radio galaxies with a prominent jet extending from
sub-pc to kpc-scales. Because of its proximity and large mass of the central
black hole, it is one of the best radio sources to study jet formation. We aim
at studying the physical conditions near the jet base at projected separations
from the BH of $\sim7-100$ Schwarzschild radii ($R_{\rm sch}$). Global mm-VLBI
Array (GMVA) observations at 86 GHz ($\lambda=3.5\,$mm) provide an angular
resolution of $\sim50\mu$as, which corresponds to a spatial resolution of only
$7~R_{\rm sch}$ and reach the small spatial scale. We use five GMVA data sets
of M87 obtained during 2004--2015 and present new high angular resolution VLBI
maps at 86GHz. In particular, we focus on the analysis of the brightness
temperature, the jet ridge lines, and the jet to counter-jet ratio. The imaging
reveals a parabolically expanding limb-brightened jet which emanates from a
resolved VLBI core of $\sim(8-13) R_{\rm sch}$ size. The observed brightness
temperature of the core at any epoch is $\sim(1-3)\times10^{10}\,$K, which is
below the equipartition brightness temperature and suggests magnetic energy
dominance at the jet base. We estimate the diameter of the jet at its base to
be $\sim5 R_{\rm sch}$ assuming a self-similar jet structure. This suggests
that the sheath of the jet may be anchored in the very inner portion of the
accretion disk. The image stacking reveals faint emission at the center of the
edge-brightened jet on sub-pc scales. We discuss its physical implication
within the context of the spine-sheath structure of the jet.
