My blip is dark flint, ~9metres below the ground at Grimes Graves in Norfolk. The site is an ancient flint mine where the flints are in three layers, the 3rd and deepest being the most sought after. Possibly the black gold of its day?
A little more here on English Heritage's web site.
A fascinating place and not where we were expecting to be today. We managed to cycle there in the dry middle part of the day and avoided getting rained on!