  The first systematic study of the serendipitous X-ray source density around
10 high z (0.24<z<1.2) clusters has been performed with Chandra. A factor ~2
overdensity has been found in 4 cluster fields, increasing to 11 the number of
known source overdensities around high z clusters. The result is statistically
highly significant (at >2 sigma per field, and >5 sigma overall) only on scales
8'X8' and peculiar to cluster fields. The blank field fluctuations, i.e. the
most sensitive measurements to date of the X-ray cosmic variance on this scale,
indicate variations within ~15-25% around the average value. The first marginal
evidence that these overdensities increase with cluster redshift is also
presented. We speculate that the most likely explanation for what has been
observed is that the x-ray sources are AGNs which trace filaments connected to
the clusters. Nevertheless other possible explanations such as an ``X-ray
Butcher-Oemler'' effect are not ruled out by present results. If the
association of the overdensities with large-scale structures and their positive
relation with cluster redshift is confirmed, these studies could represent a
new, direct, way of mapping the cosmic web and its parameters at high z.
