A Venzone mummy

For hundreds of years, a mystery surrounded the cathedral of Venzone, a small city in the province of Udine, Italy. Instead of decomposing normally, bodies buried in the tombs beneath the cathedral were perfectly preserved and still recognizable decades later, a fact which led the townspeople to periodically retrieve and commune with their dead loved ones. In modern times, scientists finally traced the source of this wonder to Hypha tombicina, a microscopic, parasitic fungus that rapidly dehydrates the bodies before decomposition can even begin. 

That's the explanation at Wondercabinet, and other sites. They have a different explanation at Virtual Tourist:

 They were already found in 1647 during works in the cathedral.  Venzone lies atop limestone bedrock. Groundwater in such regions is usually alkaline, an environment hostile to putrefaction. During floods, alkaline waters likely seeped through dirt floors in tombs 1 to 10, preserving the cadavers. Vaults 11 to 17 had sealed stone floors. Moreover, floodwaters in tombs 1 to 10 drained quickly through the porous limestone. Coffin wood from those vaults possessed just 7 percent water. In such aridity, water-soaked humans would have dried rapidly into mummies.