Last year I hired a contractor to finish my basement. I wanted to have a bar sink installed so the plumbers installed a drain pipe along a wall, jack hammered up the basement floor and connected the drain pipe to the main sewer drain. The main sewer line is located below the basement floor. I made this section of the basement a closet to hide the lid to gain access to the main sewer line, the main water lines that enter the house, the drain pipe for the bar sink. An odor is very strong in this closet area. This odor usually occurs after it has rained. The smell is not strong around the lid or pipes. The air in the closet is just a gassy smell. Any ideas?