“Where are they?”
I attended a lecture by a French scientist (whose name, unfortunately, escapes me at the moment) this past summer at the World Congress of Soil Scientists, who, by use of soil micromorphological techniques, identified the site of one of these impacts. It’s right along the seashore on the northern coast of Africa. She found shocked quartz from the impact in an arc ranging from Spain to Iraq. This was the impact, or one of the impacts, responsible for the ~2300BC catastrophe.
There is also a large, geologically recent crater in Iraq. Its discovery generated quite some interest here on FR a couple of years ago.....Blam or SunkenCiv can probably provide you with a link to pics.
The “Electric Universe” people say it could be the result of cosmic lightning, not impacts.
Lots of good topics here:
http://www.thunderbolts.info/tpod/00subjectx.htm
Chek out some of the sections under “Mythology”.