Standing water or even sewage is something that you will possibly encounter in a under home crawlspace. Mold and fungus are also most likely to be found under there as the conditions are just right for it.
Another very common danger and something most people think about if they have to enter a under house crawlspace are insects and vermin. The environment is ideal for many different creatures, some of which could be poisonous or dangerous if they are feeling threatened. This problem could be made even worse by the lack of space down there which would make a hasty retreat hard to do.
Structural problems with the house could also make entering a under house crawlspace particularly dangerous. If you suspect that the home has issues like this then its best to have a professional look at the home. If it becomes necessary that you have to go down there, take every precaution necessary and make sure someone knows where you are and can raise the alarm should you become stuck.
As we can see, all the things mentioned here are potential hazards to be aware of. If you must enter a crawlspace, be sure to wear all the appropriate protective clothing and equipment so that you will keep yourself safe and protected. Its better to be prepared for whatever is lurking down there.