  Local electronic properties of quasi-two-dimensional Pb(111) islands with
screw dislocations of different types on their surfaces were experimentally
studied by means of low-temperature scanning tunneling microscopy and
spectroscopy in the regime of constant current. A comparison of the topography
map, the maps of tunneling current variation and the differential tunneling
conductance acquired simultaneously allows one to visualize the hidden parts of
the dislocation loops under the sample surface. We demonstrate that two
closely-positioned screw dislocations with the opposite Burgers vectors can
either (i) connect to each other by the sub-surface dislocation loop or (ii)
generate independent hidden edge dislocation lines which run towards the
perimeter of the Pb island. In addition, we found a screw dislocation, which
does not produce outcoming sub-surface dislocation loops. Screw dislocations
and the hidden dislocations lines are the source of the non-quantized variation
of the local thickness of the Pb terraces, affecting the local electronic
properties.
