Why is this battery so hot, and what do I do with it?

While cleaning this morning, I noticed that the handle of a flashlight was hot to the touch. It was sitting on a carpeted floor. Upon taking the ‘D’ batteries out, one was cool to the touch and the other was really hot…almost too hot to touch.

What causes this, and more importantly, what do I do with it? An internet search hasn’t provided any answers so far.