Maybe it's time to go back to the days of hiding our life-savings in cash under the mattress.

Cybercrime expert Brian Krebs of KrebsOnSecurity has revealed there’s a subtle new method of snatching card details from ATMs that has hit the US. The information reportedly comes from a "secret-service" report released to the bank industry and ATM sub-contractor.

The technique has been dubbed “periscope skimming”. ATM skimming involves collecting people’s account information through the credit card’s magnetic strip. More conventional ways of doing this are by hidden electronics devices or adaptations that can be attached to near the cash machine’s card slot. They’re usually fairly noticeably, as the ATM will show signs it has been changed or tampered with.

This new technique uses a skinny probe that connects directly to the ATM’s internal circuit board to steal card data. As such, it’s undetectable unless you know precisely what you're looking for. These devices could remain charged for up to 14 days and could potentially store up to 32,000 card numbers.

According to Krebs, the report says there have been examples of the technology found in Greenwich, Connecticut last month, and another in Pennsylvania earlier this month.

On the plus side, the reports showed no sign of being able to get their hands on your PIN. It’s also worth considering that the criminals would need access to the inside of the ATM via a key. So, it’s unlikely an ATM attached to a bank building is likely to have the device. Krebs added that, in his experience, the comprised ATM’s are always stand-alone cash machines.

There’s more information on card skimming available on KrebsOnSecurity.com.