Scary
I was 16 when I saw The Pit on TV and despite being a mascho teenager it frightened the crap out of me.
Apart from the scary action sequences (i.e when the dead martians fell to pieces) it was the whole concept of the human race as having a "race memory" peopled with hideous fears and drives. The story might be fiction but I could see how beneath our civilised "skin" we might all have the seathing horror of the primative and animalistic lurking in our brains. That's what really scared me.