  We propose a new class of defects in QCD which can be viewed as ``embedded''
monopoles made of quark and gluon fields. These objects are explicitly
gauge-invariant and they closely resemble the Nambu monopoles in the Standard
Electroweak model. We argue that the ``embedded QCD monopoles'' are
proliferating in the quark gluon plasma phase while in the low-temperature
hadronic phase the spatial proliferation of these objects is suppressed. At
realistic quark masses and zero chemical potential the hadronic and quark-gluon
phases are generally believed to be connected by a smooth crossover across
which all thermodynamic quantities are non-singular. We argue that these QCD
phases are separated by a well-defined boundary -- known as the Kertesz line in
condensed matter systems -- associated with the onset of the proliferation of
the embedded QCD monopoles in the quark gluon plasma phase.
