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The "missing baryons", in the form of warm-hot intergalactic medium (WHIM),are expected to reside in cosmic filamentary structures that can be traced bysignposts such as large-scale galaxy superstructures. The clear detection of anX-ray absorption line in the Sculptor Wall demonstrated the success of usinggalaxy superstructures as a signpost to search for the WHIM. Here we present anXMM-Newton Reflection Grating Spectrometer (RGS) observation of the blazar Mkn501, located in the Hercules Supercluster. We detected an O VII K$\alpha$absorption line at the 98.7 % level ($2.5\sigma$) at the redshift of theforeground Hercules Supercluster. The derived properties of the absorber areconsistent with theoretical expectations of the WHIM. We discuss theimplication of our detection for the search for the "missing baryons". Whilethis detection shows again that using signposts is a very effective strategy tosearch for the WHIM, follow-up observations are crucial both to strengthen thestatistical significance of the detection and to rule out otherinterpretations. A local, $z\sim0$ O VII K$\alpha$ absorption line was alsoclearly detected at the $4\sigma$ level, and we discuss its implications forour understanding of the hot gas content of our Galaxy.

X-Ray Absorption by the Warm-Hot Intergalactic Medium in the Hercules Supercluster