This is probably the most frequently asked question we get from our customers. The truth is it can come into the crawlspace in various ways but it always starts from above... the rain. From there:
1:  In heavy or long consistent rain, lawns can become saturated and so many customers believe the answer is doing exterior french drains. While french drains are good for slowing down water that is coming downhill, heavy rain spreads out beyond and under french drains. With the footing of a foundation anywhere from 3ft below the surface of the exterior dirt line and a french drain sitting typically 1-2' below ground level, this does little to stop the water.
2:  Bad Rain Drains. A roof takes on an average of 623 gallons of water per sq ft during rainfall. You can imagine how much falls during a heavy downpour. So, if your downspouts are not sufficiently moving the rain well away from your property, that water will saturate the area around your foundation and flood your crawlspace. While a proper drainage system and sump pump can help, the best way to handle downspouts is to extend the downspouts under ground to a safe distance away from the foundation.
3:   Water Tables.  If you live in an area with high water tables, as they rise, so will the water in your crawlspace. If you have a stream or pond nearby you probably have high water tables but even without an obvious water source, some areas can suffer from an underground springs that will cause the same issue.