Some of mines
#1
Going down the Hecate Research Outpost for the first time (never been there yet and I was on a DOMINATING no stealth run),
seeing webs everywhere and only seeing 1 Greater Coil Spider pass and exit my line of sight from far away. I exited the place
like Usain Bolt breaking a new world record.
#2
Finding a Plasma Sentry walking near Blaine's shop on DOMINATING while I was level 3. I got scared AF.
I thought it would always be there on DOMINATING at first.
Another time finding the Arsonist on the same map (I found a way to beat him at level 4 with traps and get a over the top shield from the get go).
#3
Getting your first surprise meeting with a Crawler/Death Stalker or those pit holes where you find a bunch of them hidden. Like that spot in Lower Passage where there is a strong group of Lurkers guarding a ladder going down (near the first accessible Electrokinesis Imprint: Blueprint).
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Did anybody ever draw a vertical map of the Underrails from Deep Cavern up to the unexplored surface ? I always wanted to see that.