While stronzaccio knows way too much about the process:blog.cleveland.com/andone/2008/10/the_flash_p...

This type of crime tends to be done by organized gangs of professional thieves. The same types would steal safes from large stores and work on them at their leisure in a warehouse or garage somewhere.

The problem isn't getting into the ATM; it's getting rid of the thing after the money's been removed. My guess is they get shoved into the river from the back of a flatbed truck. It's probably safe to assume that the group of thieves includes someone who's worked on ATM machines and understands how they're put together - just as gangs of safe thieves usually included a locksmith or someone with professional experience of the workings and construction of safes.