Oceancoversalmostthreequarters(71%)ofthesurfaceoftheEarth,andnearlyhalfoftheworld'smarinewatersareover3000mdeep. 　　Thisglobal,interconnectedbodyofsaltwater,calledtheWorldOcean,isgenerallydividedbythecontinentsandarchipelagosintothefollowingbodies,fromthelargesttothesmallest:thePacificOcean,theAtlanticOcean,theIndianOcean,theSouthernOcean,andtheArcticOcean.Thelastoneisoceanographicallybetterdescribedasamediterraneansea,however. 　　Smallerregionsoftheoceansarecalledseas,gulfs,straitsandothernames. 　　Geologically,anoceanisanareaofoceaniccrustcoveredbywater.OceaniccrustisthethinlayerofsolidifiedvolcanicbasaltthatcoverstheEarth'smantlewheretherearenocontinents.Fromthispointofview,therearethree"oceans"today:theWorldOcean,andtheBlackandCaspianSeasthatwereformedbythecollisionofCimmeriawithLaurasia.TheMediterraneanSeaisverynearlyitsown"ocean",beingconnectedtotheWorldOceanthroughtheStraitofGibraltar,andindeedseveraltimesoverthelastfewmillionyearsmovementoftheAfricanContinenthasclosedthestraitoffentirely,makingtheMediterraneanafourth"ocean".(TheBlackSeaisconnectedtotheMediterraneanthroughtheBosporus,butthisisineffectanaturalcanalcutthroughcontinentalrocksome7000yearsago,ratherthanapieceofoceanicseafloorliketheStraitofGibraltar.)