As Tom Leek points out, it's not safe, it's about reacting to tampering. Nothing is inherently safe, so the key is to have the money inside rendered useless.

As Tom Leek points out, it's not safe, it's about reacting to tampering. Nothing is inherently safe, so the key is to have the money inside rendered useless. Be it broken-glass, seals, radio/gps, there are probably many other ways of detecting non-normal use (given ATMs is usually bolted). Add that to the fact that it's incredibly risky and difficult to physically remove. What you would end up with would be useless or marked money. So, it's not 100% secure, but it is a futile target. Off topic: You can compare this to cryptography. Given an attacker has (physical) access and knows your system, it should still be impossible to crack.