Thank you, this is all very useful!

Yes, I didn't know WTF was there but over the years it had grown beyond annoying , becoming so painful I couldn't tolerate it. I thought perhaps something (a splinter, piece of glass or steel, etc.) had become embedded in my foot. I was determined to dig it out. I'm tall and not flexible so I cannot easily see all of the bottom of my foot. But I can reach it.

The callus was surprisingly small (~1/2") and came out in one piece after about 10 minutes of work. Nothing embedded. No bleeding, just a lot of knife-wiggling. The bottom of the foot is really tough!