These are cavernous rock structures that I found on the site in Sokoto.

Now, I was taken by inspector Brian Cantera, of the Royal Canadian Mounted Police, to a cavernous warehouse east of Vancouver to see some of the goods which are regularly confiscated by the RCMP from the smugglers who are sending it, of course, down south to the United States where there is an insatiable market for B.C. Bud, as it's called, in part because it's marketed as organic, which of course goes down very well in California.