A two-phase cooling system operated at atmospheric pressure. A reservoir containing cooling fluid has a stack that is vented to the atmosphere. The stack is shaped to allow condensation of substantially all of the cooling fluid in vapor form entering the stack. Condensation may be enhanced by cooling the stack, such as with flowing air along the outer walls of the stack or placing a thermoelectric device in contact with the stack. The system provides high thermal capacity but is easy to use and service.