Building work has been going on to stabilise the ancient monument of Silbury Hill in Wiltshire for six months but before sealing up the tunnel, media were allowed inside it.
Parts of the 4,400-year-old Neolithic site were thought to be collapsing because of the tunnels dug by archaeologists over many centuries - the last of which was created in 1968.
As engineers worked to stabilise the monument, archaeologists have tried to unlock the site's ancient secrets and find out how, why and when it was built.
Discoveries include medieval postholes on top of the hill and iron arrowheads, indicating there may have been a huge military building there during the Saxon or Norman periods.
One theory is that the top of the hill was lopped off around the time of the Battle of Hastings or even earlier.
From next week, the tunnel will be repacked with chalk to stabilise the hill for future generations to appreciate.