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I don't think ive ever spotted this invisible floor before... its right above a deathbarrier so if you dont pay attention to the death animation you will be none the wiser! The way i discovered it was i was messing around right above it, and i fell down (as you do). I noticed the animation was off to me, so investigation time happened!
First, we need to get to the low levels of the upper levels... Right in front of where you exit the elevator, but across the gap, so we need to jump from above. Thankfully the invisible floor extends just out of the building so we can walk on it :P
At some point tho you wont be able to keep walking along the ledge. There is an invisible wall now...? idk, but you cant walk around it as the ledge is too small, and you cant grab it either. In fact, you cant grab anything around this area. If you try to jump over the wall, you will land on a slippery surface (wtf is this place??), so you need to kick when youre falling onto it. Kicking on slippery surfaces stops you, and can change your direction. In this case, we dont need to change our direction tho
Now lets get inside the lower levels of upper levels! Normally you could crawl in through an opening here but you cant really grab it as everything else... Instead, walk slowly towards it, and with some luck youll vault over the board that sticks out and voila! youre almost in!
The invisible floor itself covers a quarter of the floors total area... just in this corner to the right of where you entered. You can actually walk over to the corner and its still present there too, but i didnt show that in the video
To get back out, while we dont have to worry about the slippery surface anymore, the wall is still blocking us. BTW it also blocks us from falling directly away from the wall in this area... this place is weird...