Now here’s a fun fact that almost any seasoned NES player back in the day was well aware of, but has since been quietly forgotten: the already massive world of Metroid was even bigger than first glance.
Contained in the game are various closed off rooms that can only be accessed by exploiting a glitch. It’s somewhat tricky to perform, but can be executed after a bit of practice; simply shoot open a blue door and stand immediately next to it without going inside. Once it closes, Samus will be stuck inside, so at that point, rapidly switch between morph ball to standing up make your way upward. You won’t always find a hidden section, but they are everywhere.
Not sure what the official story is, though many have theorized that they were supposed to be part of the game and were cut off due to time and developmental restraints. In the end, it makes an already creepy game even more so, since anyone could find him or herself permanently stuck or the hapless of victim of even more glitches.
There’s a few videos floating around that demonstrate the glitch in action, but this might be the best one overall, mostly because it was performed in the actual game (not emulated, not that it makes any difference really) and captured on a VCR:
There used to be a web page that detailed every hidden section, but I can’t seem to find it, and am afraid it got washed out when the plug got pulled on GeoCities. Bummer.