Land during day, start flattening ground to put a portal down, angry troll with super hearing comes running because i touched his precious soil, no spawners but greydwarves come from every direction, agro a bunch more running away (to preserve boat). Fortunately some of the hoarde got distracted by skeletons and non-player wooden doors. By the time I could get back and chuck down a portal it was already night time.

But, that's the game. You don't get to just only attack mobs when you feel like it. Wait until you're trying to stealth recon your first swamp and non-stop greydwarf spawns keep agroing every zombie in 50m. I know there's no spawner there either, in fact except for the copper deposit it's almost a straight swamp/mountains transition.