Silbury Hill (near Avebury in the UK) is the tallest (c.30 mtrs) prehistoric man-made mound in Europe. It was built around 2470 - 2350 BC, mainly from chalk boulders, clay and earth. Since it must have been predominantly 'hand-built', it's been possible to estimate that its construction would have taken around 4 million man-hours.
It has been extensively investigated for many years, most recently using ground-penetrating radar and other technical methods. but no clues have so far emerged as to its original purpose.
Suggestions include ritual use of some kind, as an observation platform, as a monument etc etc. (no central burial site has been discovered).
Further reading : English Heritage, Silbury Hill.