1. A data center cooling system, comprising:a first information technology (IT) room having a first set of a plurality of electronic racks, each of the electronic racks in the first set containing a plurality of IT components therein, wherein at least a portion of the IT components are attached to cold plates;
an external cooling unit positioned above the first IT room, the external cooling unit including a heat exchanger, configured to provide cooling fluid and to exchange heat carried by the cooling fluid;
a supply line to receive the cooling fluid from the external cooling unit and to distribute the cooling fluid downwardly to each of the cold plates in the first IT room to exchange heat generated by associated IT components attached thereon, wherein the cooling fluid evaporates into a vapor state from a liquid state during the heat exchange; and
a return line to return the cooling fluid in the vapor state upwardly back to the external cooling unit, which transforms back to the liquid state due to heat exchange within the external cooling unit;
wherein the external cooling unit includes a first fluid tank external to the heat exchanger and fluidly coupled to the heat exchanger to receive cooling fluid and to store sufficient cooling fluid to provide sufficient liquid pressure to distribute the cooling fluid downwardly through the supply line.