So when do ATM's get stocked?

I realized that I've never seen someone fill the machine with cash, nor have I ever seen an ATM that was out of money. Yeah people deposit cash too but in a given day I'm sure more cash is taken out of ATM's than there is cash put in.

So either the banks have ninjas stock the machines, or they have a secret underground pipeline full of $20 bills.

Febuary 31 every year.

Underground network of tunnels.

They use a secret network of tunnels where Dwarves with mining carts go back and forth from bank to atm and then they stock them through tubes. True story, and you heard it here first.

I'm pretty sure there are cartridges inside the ATMs that house plenty of money, and I assume that if they were to run out, they would be refilled, thanks to the ninja workers. I'm probably wrong, though.

Here is your example.

You thinking of committing a crime?

Seriously thoe, it's Febuary 31 every year. 

I now have proof of activity beneath the tunnels! Well this is the best I could find anyways:

Hey, I never thought of it like that,

I've seen ATMs run out of cash before, its a weird WTF moment when you get that error message.

Living a couple blocks from an ATM, I can also say they do actually replenish them. Its an armored car with two armed guards and one designated transaction person with the key that does the job. They are super quick about it too and actually doesn't come at any regular intervals.

Liked stated it is usually an armored car that goes around and fills them.  There is not set schedule but they are usually changed in the morning to avoid any possible unfriendly encounter.

Now privately owned ATMs are changed by the owner of the machine.  This tends to be a guy with a big sack of money but he is normally in normal clothing and doesn't tend to draw attention to himself or the fact that he has a ton of cash on him.