The 4 DNS servers you can use for your domains, are apread globally for performance and redundancy, here is the complete POP list
Amsterdam, Netherlands (ns1, ns2)
Berlin, Germany (ns4)
Frankfurt, Germany (ns3, ns4)
Sao Paolo, Brazil (ns4)
Dallas, United States (ns1, ns2)
London, United Kingdom (ns3, ns4)
Los Angeles, United States (ns3, ns4)
Moscow, Russia (ns4)
Miami, United States (ns3, ns4)
Prague, Czech Republic (ns4)
San Jose, United States (ns1, ns2)
Seattle, United States (ns1, ns2)
Singapore (ns1, ns2)
Sydney, Australia (ns4)
Ukraine (ns4)
Washington, United States (ns1, ns2, ns3, ns4)
Hong Hong (ns1, ns2)
Stockholm, Sweden (ns4)