9-Ton Slab of Glass Found in the Cave of Beit Shearim

What is a 9-ton slab of glass doing in the middle of a burial cave in Beit Shearim, Israel? And why does such a small town needs a wall, what were they protecting?

Beit Shearim is an ancient Jewish necropolis where Yehuda Ha Nasi, the head of the Sanhedrin, was buried. This catacomb cave dates back to the second century AD and is one of the greatest archeological sites in Israel.