Some 2.5 GHz dual processor and all 2.7 GHz dual-processor and the 2.5 GHz quad-processor variant had a liquid cooling system that consisted of a radiator, coolant pump, and heat exchangers bolted to the processors.
The liquid cooling system fits into the case where the heat sinks would normally go, so there is no easy way to distinguish the liquid-cooled versions from the air-cooled, although most, but not all, of the liquid-cooled machines have a sticker inside warning about the possibility of leakage.