I'm not a real expert in this area, though I have some experience of fire-fighting pump installations both on- and offshore. I've not seen an underground one, but agree the normal concern is about leakage and groundwater pollution. But that's for buried tanks, rather than ones in a separate room. If the tank is suitably bunded and the bund is regularly inspected, that should not be a problem. Because of the very limited access, it would be foolish to install much equipment, as that increases the number of entries required for inspection and maintenance. However, it's also foolish to design the chamber as a confined space, so are you sure it's not intended to have fresh air circulation supplied externally? So I suggest any pump needed to drain the bund should preferably be an air-operated portable one brought it when/if needed. Re ATEX, etc. Diesel has a high flash point, so normally rated electrical equipment isn't needed. Similarly, if there's a fire in a fire pump room then something is very seriously wrong and the normal design philosophy is just to let the pump keep running until destruction, rather than add extra equipment to try to protect it. So it's also very important not to allow combustible materials to accumulate, hence the need to inspect the bund and pump out any spillage.