Sentence:  This shelter could be built in regions where <WSD>water</WSD> or rock is close to the surface , making it impractical to build an underground shelter . 
Meaning of the word marked with <WSD> tags:  binary compound that occurs at room temperature as a clear colorless odorless tasteless liquid; freezes into ice below 0 degrees centigrade and boils above 100 degrees centigrade; widely used as a solvent