They're hollow hills that generate on the map. You'll know them by their entrances: Cobble around a door-sized entrance blocked by iron bars and topped with chiseled stone brick. Be careful, though. They're full of monsters (being basically a kind of dungeon, they have a couple of spawners each) and the node generates wisps, of course. They usually have good loot (including knowledge fragments and thaumium), thankfully, so they're worth a visit even if you don't use the node.
