I really should erect a larger fence around it in the short term since if you fall it in your inventory will be lost.
It is know to us and I've filled it twice from a backup but we are still trying to determine the cause and why it keeps reappearing. It is essentially the 0/0 chunk which is the new maps original spawn (our spawn was moved to suit the location of the copied over buildings). Taking the repair of this slowly and with a lot of testing given the region file for this chunk is the most densely populated.
Any luck with trying any of these?
http://www.reddit.com/r/Minecraft/comments/egpr7/how_do_i_fix_corrupted_areas_on_a_smp_map/