Strange how only some people get this eerie feeling and refuse to enter into sites....whilst their companions carry on undaunted...I still think its a learned response instead of an actual sense...or perhaps a claustrophobic reaction as someone pointed out. I know several people who wouldn't enter into a ring of stones if there were horses there(even 1 horse)....and certainly a lot more if there was an abundance of spiders. Perhaps the mere thought of something like birds, spiders or rats in an enclosed monument, would be enough for some even though none were evident.