I will readily admit that, as a kid, Olivine Lake scared the crap out of me. The object of the stage was to help Pikachu find five Poliwag who were lost in a maze of a boardwalk, which sounds sweet of Pikachu and not at all traumatic (other than the usual Pikachu-induced traumas). However, the way my young mind saw it, there was more than enough nightmare fuel. It was foggy and you couldn't clearly see where you were going, so zombies could quite possibly rise up out of the lake to eat the player character without you even knowing it. (I had an overactive imagination.) It played significantly less happy music than any other stage in the game, which equated to death. The way the game phrased the mission made it seem that the Poliwag you didn't find before the day was over would just up and die, their bodies never to be recovered like one of those depressing missing child stories, and the mother Poliwhirl wouldn't even care and just forget about her poor dead children floating in a bush somewhere out on the marsh/lake. And last but certainly not least, there were Haunter that constantly popped up behind you to scare off all the Poliwag you collected.
The stage is significantly less creepy nowadays, though. Vicki and I easily spot Poliwag halfway across the lake and marvel about how much easier this was now that we were older and less easily creeped out. Pikachu, of course, is just meandering along the boardwalk as if we were in a lovely spring field rather than a creepy lake and had nothing better to do than look at the sky, despite being the one to sign us up for Poliwag hunting in the first place. It completely ignores the group of four Poliwag walking so closely behind it that they're shoving their heads and faces inside Pikachu's and each other's butts. Only one more to find until we can all go home—hip, hip, hooray!
"We haven't checked for Haunter in a while," I comment as Vicki moves the player character along the boardwalk. We tend to get a bit nervous on the final stretch. It really sucks when you're this close to being done and a Haunter ruins it all, so Vicki and I made it a habit to check the surroundings every once in a while.
"You're right," Vicki agrees. Pikachu and its duckling-Poliwags are walking in front, so and there's nothing we can see coming from that area, so Vicki puts the camera into view mode and looks behind us.
To get the full scope of what we saw, imagine that your monitor is a TV screen. Picture it completely purple because the Haunter is so close behind you. Then paste huge Haunter eyes and its grin onto the purple screen, imagine it wailing "HAUUUUUNTER!" and you'll be experiencing in an imaginary sense exactly what was behind us.
Needless to say, we practically went into cardiac arrest when greeted with that and screamed. I would tell you exactly what I was thinking at the time, but it was mostly various words that would raise the rating of this story significantly. Not only that, but Pikachu got startled as well and screamed its name and all the Poliwag started yelping "Poli! Poli! Poli!" as they ran away in different directions. I'm fairly certain a neighbor who happened to be outside at the time glanced at our sliding glass door wondering why two teenage girls were screaming like they were being murdered.
"Holy—oh my god—what the ****?" I finally shouted when I had recovered enough that I could breathe and didn't think my heart was actually going to stop.