Silbury Hill stands almost forgotten on the Wiltshire countryside. The largest prehistoric mound in Europe, the Hill is nearly eclipsed by the close proximity of the megaliths of Avebury. Silbury Hill, however, is also a man-made structure from the distant Late Neolithic period – and it’s almost as big as some of the Egyptian pyramids.

Despite its relative anonymity, Silbury Hill is just as mysterious as the iconic circle of stones that it neighbors. Its mystery lies in its reluctance to reveal its purpose: Historians and archaeologists have puzzled for centuries over why it was built, but excavations from the 1680s to the present day have all failed to fathom its function. They’ve unearthed nothing from inside the hill but clay, gravel, flint, and vegetation.

Some have suggested that the man-made mound is the tomb of the legendary King Sil. Seventeenth-century antiquarian and Silbury Hill excavator John Aubrey once noted that,“No history gives any account of this hill; the tradition only is, that King Sil or Zel, as the countrey folke pronounce, was buried here on horseback, and that the hill was raysed while a posset of milke was seething.”

Recently, some light was shed on this enigmatic site when a letter from Edward Drax concerning his 1776 excavation of Silbury Hill was discovered in the British Library. Drax described a 40-foot “perpendicular cavity,” six inches wide, in the base of the hill. As wooden fragments have been found at the Hill, it has been suggested that this cavity may have held an oak tree or a “totem pole.” But Drax was not so fast to jump to conclusions and only stated that “something now perished must have remained in this hole to keep it open.”

The body of an ancient king? The mystery of Silbury Hill continues.