A replica of Zongazonga's Castle can be found after the post game. While tecnically a Dig Site, in the sense you can use your sonar and pickaxe, you will find no fossils. Once there, the only way off the castle is to use the VMM and teleport, which will bring you back to Ribular Island. At the site, you can talk to a Staff Member, who will allow you to battle Zongazonga once more, except Zongazonga and his Z-Rexes will be Rank 20. The battle costs 3,000 G, and beating Zongazonga the first time will give you all of his Zombiesaurs.