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Kertesz line and embedded monopoles in QCD

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.