It's not because "the bit might still be visible under a microscope" - it's because of something called (IIRC) "intra-track data". Basically when a hard drive

It's not because "the bit might still be visible under a microscope" - it's because of something called (IIRC) "intra-track data". Basically when a hard drive stores the data on the physical platter, it writes it as a series of concentric tracks. These tracks are spaced very close together. When data is written to the drive, the magnetic regions that are written on the track can "bleed over" to the intra-track regions, leaving a faint trace that with the proper equipment ($$$$) can be read, even if the tracks are all zeroed. Think of the magnetic regions as not being (say) "spikes", but rather "hills", where the greatest magnetic intensity is at the peak, tapering off as it reaches the edge/base of the hill. That energy might be small, but it is there. By overwriting each track multiple times with varying data patterns, these leftover tale-tell magnetic leftovers are essentially pushed into the noise of the rest of the data, and are effectively "wiped". But you have to write the pattern multiple times, over all the tracks. This effect has "recently" been used to an advantage (depending on how you look at it) for something called an "SMR Hard Drive" - or "Shingled Magnetic Recording": https://en.wikipedia.org/wiki/Shingled_magnetic_recording https://en.wikipedia.org/wiki/Shingled_magnetic_recording In this kind of hard drive, the tracks are purposefully placed very close together so that the magnetic regions purposefully overlap, allowing for a higher density on fewer platters (so, larger drives in a smaller space for a lower cost). However, they tend to be slower - and not recommended for purposes like RAID NAS or anything where you want speed and proper data retention (my understanding, at least).