  Suspended graphene is difficult to image by scanning probe microscopy due to
the inherent van-der-Waals and dielectric forces exerted by the tip which are
not counteracted by a substrate. Here, we report scanning tunneling microscopy
data of suspended monolayer graphene in constant-current mode revealing a
surprising honeycomb structure with amplitude of 50$-$200 pm and lattice
constant of 10-40 nm. The apparent lattice constant is reduced by increasing
the tunneling current $I$, but does not depend systematically on tunneling
voltage $V$ or scan speed $v_{\rm scan}$. The honeycomb lattice of the rippling
is aligned with the atomic structure observed on supported areas, while no
atomic corrugation is found on suspended areas down to the resolution of about
$3-4$ pm. We rule out that the honeycomb structure is induced by the feedback
loop using a changing $v_{\rm scan}$, that it is a simple enlargement effect of
the atomic resolution as well as models predicting frozen phonons or standing
phonon waves induced by the tunneling current. Albeit we currently do not have
a convincing explanation for the observed effect, we expect that our intriguing
results will inspire further research related to suspended graphene.
