The squigly line to the left of the pit is a curtain of magical darkness (I probably got this idea out of one of the 2e Undermountain boxes).
The curtain itself is one of my favourite things to include in a random corridor in a megadungeon (and change whatever is behind it). Often, the curtain also blocks sound in my games. I usually manage to restrain myself and not overuse it though.
If you've been reading the comments in this community, an idea that has been brewing for some time is getting a bunch of these micro-maps a stitch them into something bigger.
I recovered many tiny pieces of paper and started making even smaller maps. I hope these help to fill in gaps when assembling a larger dungeon.