This may be old news to some of the more experienced here but I had not seen it in 40 years of being around trucks.
I had a new set of tires put on the bus today and when the tire guys were dismounting the first one they could not get it off the rim. After about 30 minuets they decided they were trying to take it off of the wrong side of the rim. They turned it over and in about 5 minuets they had the old tire off and the new one on. Two of them worked like that and then they had to do the other 4 from the other side. The tire guy tried to show me what the difference was in the rim sides but I could not see it. Just a heads up if you are trying to get a tire off and it will not come off, turn it over and try it from the other side. I did not know it mattered which side you took it off from.
It is called "drop center". The valley is there to facilitate tire installation.
http://arrc.ebscohost.com/ebsco_static/repair-tips/8852.htm#8852CH24_Wheel_Construction.htm (http://arrc.ebscohost.com/ebsco_static/repair-tips/8852.htm#8852CH24_Wheel_Construction.htm)
The drop is in different places depending on rim design and manufacturer.