The other problem you have when the diesel fuel tank is below the engine is getting the fuel up from the tank to the engine. The height (when tank fuel level is at its lowest....!) from lower tank level to engine has to be less than the engine pump lift level.
The way round this is to put a day tank in the engine room (bunded) fed by an electric pump from the main tank. Then you are increasing the complexity and reducing the reliability.
itsmoked - natural gas can be used, but the engines are more complex (less reliable [before any one jumps on me, this is because they are usually designed for continuous operation and have more protection than a simple diesel engine]) and the gas gives an additional safety issue..... Beacause the gas is usually piped from the utility and not stored on site, then there is always the risk of a supply interuption.
By the way, avinolga, do you mean underground as in buried in the soil (a real no-no in most places now, as others have said) or do you mean in the basement? Depending on your local codes, you MAY be able to put a bunded tank in the basement. There are so many issues, you really need a local expert to advise you