Reddit user harritaco discovered something rather unusual about the iOS devices used at their place of work. iPhones and Apple Watches stopped working unexpectedly, completely locking up and recovering only days later, sometimes suffering long-term harm.

The failures appeared to coincide with the installation of a new MRI (magnetic resonance imaging) machine. MRIs use powerful magnetic fields and helium-cooled superconductors, and something about the presence of this new machine was upsetting the Apple hardware. That magnets can be a problem for electronic devices is no big surprise—they can damage magnetic media, confuse compasses, and induce electric currents in harmful ways—but surprisingly, it's not the magnets that seem to be the problem this time—it's the helium.

The iPhone user guide warns that proximity to helium can impair functionality and that to recover, devices should be left to air out for a week or so in an environment far away from the rogue helium. Harritaco discovered that, during installation of the MRI machine, some 120 litres of liquid helium leaked and vented into the environment. This created a relatively high helium concentration, and any Apple hardware exposed to that helium stopped working.

To test this hypothesis, harritaco conducted some experiments in which an iPhone was put in a sealed bag of helium; after a few minutes, it stopped working.

This leaves one big question: why? Redditor captaincool offered a plausible theory. Smartphones contain microelectromechanical systems (MEMS): tiny mechanical systems that are integrated into chips. These mechanical systems are used for things like the gyroscope and accelerometers. These devices operate in tiny cavities within the chip (they need space to flex and vibrate), with those cavities either evacuated to create a vacuum or filled with a gas with known properties.

This means that the MEMS devices have a seal to plug the gap where the air was pumped out (or another gas pumped in). While those seals are able to keep most air constituents out, they're permeable to the very tiniest molecules. In particular, both diatomic hydrogen and monatomic helium can permeate the seals. At normal atmospheric concentrations of hydrogen and helium, this is no big deal, but, when exposed to high helium concentrations—such as the concentrations experienced by harritaco—enough helium could diffuse across the seal to render the MEMS components non-functional (at least temporarily, until the helium escapes).

Our guess is that a MEMS oscillator used to generate a clock signal for the phones is misbehaving after exposure to helium, hence the system-wide failure.

That's probably the weirdest way to make a smartphone stop working that we've heard. As for why non-Apple devices appear to escape without harm? They might use different seals or perhaps aren't using MEMS devices in such critical roles.