DEEP in the El Castillo cave in northern Spain, up a side passage off the main pathway, there is a small painted panel most visitors would walk straight past.Created some 17,000 to 15,000 years ago, it shows five red signs, each shaped a little like a bell. A natural stone ledge runs beneath them like the lip of an altar. For over a century, archaeologists have simply catalogued these as “bell-shaped signs,” filed under the vast heading of ice age marks we don’t know the meaning of.