So, a couple of weeks ago I bought a dehumidifier for our unfinished
basement - the walls are poured concrete. After a couple of days I
noticed a horizontal dark line about two inches wide stretching along
one of the walls. It starts in a corner and just ends a few feet along
the wall. It is under the level of the soil outside - the ground
outside is well graded away from the house, and it hasn't
rained much recently. The line does not feel wet to the touch. I assume
this has something to do with the humidity in the basement decreasing
relative to the moisture in the ground outside/in the walls?