Imagine yourself running through dark corridors on an alien planet. For a moment, there’s no sound but the clank of your metal boots on passageway floors carved from stone, and the slow drip of water somewhere in the distance. You know that won’t last long. You’ll meet an alien life form eventually, and the encounter won’t go well. But in the meantime, you’re stocking up on supplies. Energy upgrades, for starters, and maybe the odd new ability or two that will change everything.

You’re playing Metroid, which debuted on the Famicom Disk System in Japan 40 years ago today. At the time, it was just one more very good adventure title from a studio in the midst of building a reputation for such things. Over the years, however, Nintendo’s bold experiment would inspire numerous imitators and become known as the basis for something we love more than ever: the Metroidvania.

The original Metroid introduces players to a space bounty hunter named Samus Aran, who works to recover important organisms known as Metroids that have been stolen by enterprising Space Pirates. In the wrong hands, which is where they now find themselves, those life-draining entities might be used as devastating biological weapons to threaten the safety of the entire galaxy. To defeat the thieving pirates and their schemes, Samus must disable their defenses, which are operated by a powerful intelligence system called Mother Brain.

Although the plot is interesting and established lore that eventually expanded to serve as the basis for numerous additional games, including the Metroid Prime quartet, Metroid’s true appeal lies in its gameplay. Nintendo’s development team created an action game set within a non-linear world that offered multiple paths to success. In some ways, it felt like a more action-packed take on the sandbox style The Legend of Zelda offered a short time earlier, now viewed from the side. However, there were no “levels” to speak of, just a hostile environment to wander and conquer on the way to the final showdown.

At the onset, Samus Aran has few weapons at her disposal. The task placed before her seems almost insurmountable. In those early days of the medium, the game’s scale felt absolutely incredible. Developers were still releasing new titles that only occupied a single screen, or featured discrete environments that didn’t linger long at all. Then there was Metroid, scrolling every which way and presenting constant twists and turns, to say nothing of passages you could find but not reach… yet.

Locating the entrance to a new area that you can’t access until you return with more firepower or new abilities appeals to gamers on a level that defies description. Metroid discovered that loop decades ago, combining it with an interesting world that players would want to return to regularly. A little peashooter is eventually joined by more powerful ordinance, such as missiles and even an ice beam that freezes enemies in place. More importantly, Samus learns to roll up into a tight ball that can pass beneath low ledges to find hidden chambers, and to drop bombs that detonate around unsuspecting foes or even make it possible – with careful timing and lots of practice – to ascend to loftier heights than ever before.

Players tested the limits of those new abilities, thrilling as they unearthed alternate routes and discovered new ways to engage with the hostile planet of Zebes. And then there were the speedrunners, who did all of that but even faster. The game encouraged such efforts, rewarding resourceful players who reached the end most quickly with a glimpse beneath the suit. (Believe it or not, the reveal that Samus was a woman was a major twist by 1986 standards — the game’s instruction manual even referred to her using male pronouns.)

It’s possible to complete Metroid in fewer than 20 minutes even without using glitches, but players won’t manage such feats on their first run, especially without a walkthrough by their side. Because Zebes might occupy players for many hours, Nintendo implemented a password system that let them resume their adventure at a later date. The setup also allowed players to swap passwords with friends, if they wished, which turned things collaborative. One gimmicky password many players committed to memory let them enter “Justin Bailey” as the top row of characters, then a whole line of dashes on the next one. People still talk about it.

Metroid was a big deal in the early days following the NES launch in North America. We obsessed over its sprawling corridors and secrets. We wrote fan mail to Nintendo Fun Club News, gushing over our love of the game. The game was a monster hit back when it was still perfectly respectable to sell only a few hundred thousand units, rather than tens of millions.

Eventually, alongside later games like Super Metroid and Castlevania: Symphony of the Night, the original Metroid helped to inspire hits such as Hollow Knight, Blasphemous, and numerous other modern favorites. If you love those newer games, you owe something to the imperfect but unforgettable experience Nintendo launched four decades ago, with Metroid. (If you have a Switch Online subscription, it’s available to play through the NES library app.)