If you want to get kicked out of the place, call the city. If you don't, don't call. it's kind of weird to have a tank that large under the bathroom floor, but it's pretty routine to have a tank in the basement which gets regularly pumped up into a sewer line. Are there city sewers on the street? Any explanation as to why the tank is there? The smell could simply be the result of a plugged vent. Clean the vent and the smell might go away.

If you really want to find the bottom, jump down in there and dive deep.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13