Soil Gases in the crawl space can be more deadly than you might think. Radon Gas is the second leading cause of lung cancer in the US.
Radon Gas must be evacuated properly to protect your family. The elderly and children are the most susceptible to the effects. but it does affect everyone. Radon is a tasteless, odorless gas that comes from deep in the ground. The only way to know you might have a problem is to do a radon test.
Other Soil Gasses
Aside from Radon Gas, there are many other soil gasses that come from the earth that are a natural part of the environment. These gasses, like ammonia and methane, can give off an unpleasant odor. There are options to mitigate these soil gasses much like you would with Radon. However, ammonia more specifically can be mitigated by an additional way if that is the only soil gas problem you have. Ammonia in the soil will make your crawl space smell like "Cat Pee". The initial suspect will be a cat, but that is most often leads to a dead end as a solution. The culprit is most often the soil in the crawl space being high on the pH scale. Ammonia is not present in acidic soil. Here is an article on the subject of "Cat Pee Smell in the Crawl Space"
I hope this helps, and don't forget to download the crawl space encapsulation inspection checklist to make your project go as smoothly as possible.