So we bought a house a couple of years ago (the house is approximately 12 years old, weather-proofing is wonderful, no mold or moisture issues), but probably about three or four times in the last two years, it's smelled like a dead animal or sewage (my man thinks it smells more like sewage; I think it smells more like a dead animal) under our master bedroom.

The master bedroom is on the ground floor, directly above a crawlspace that is only accessible from outside. We have a bathroom attached to our master bedroom, but on the opposite side of the room from the smell. The smell seems to be coming from the floor, so it's obviously something in the crawlspace. The smell gets completely overwhelming when it happens, but then will eventually go away after a couple of weeks. It tends to coincide with heavy rains, but we live in a very moist area (Pacific Northwest) and it doesn't happen with -every- rain.

Since it's happened about four times, we have no idea if it's some strange sewage line issue only with really heavy rains, or if we're just really unlucky and have a very inviting crawlspace for dead/dying animals. We do live right next to a greenway, so there are a lot of animals (rats, squirrels, raccoons, etc.) in the area.

Upon a cursory look in the crawlspace, we weren't able to find any dead animals, but there is so much insulation, etc. under there, it's really hard to be sure if we've been able to check everything. The crawlspace goes under 1/4 to half of our downstairs area, so there is a lot of room for dead things if animals do get down there.

What can I do to help identify the location and cause of the smell? How can I prevent it in the future?

Smells are hard to track down because air currents move them around. Since this recurs, I don't think it is a dead animal. You could have a cracked drain or vent pipe. The Pacific NW is noted for dense high humidity levels which are heavy per se and sink. The dense air could be pushing down a vent pipe and forcing sewer gas smells out through a crack or loose fitting. Carefully look at all waste pipe fittings in the crawl space or any wet spot.