"Leaking HD 625 button cell battery". Apparently a mercury-based cell (and no longer made). "(This *was* the light meter battery in my Zenit TTL, until I unpacked it today- it looked like it might have forced the battery cover off(?) Could be around 30 years old- I don't recall ever replacing the battery from the time I bought the camera secondhand in the mid-80s until I stopped using it in the late-90s.)"