"Since then, scientists from the Miquel Crusafont Institute of Palaeontology in Sabadell have unearthed nearly 12,000 fossils of primates and other animals that lived during the Middle Miocene era - between 14m and 8m years ago - when the area was covered by tropical rainforest and populated by the precursors of today's elephants, antelopes and monkeys."
If its been a dump for hundreds of years, the layers and layers of refuse would provide the perfect protection for the site as civilization and industry grew all around it.
An unintentional protector for the treasure underneath.