Standing water or wet soil in your home's crawlspace can come from a variety of sources. These may include but are certainly not limited to the following.
(1) A leaking freshwater supply line, usually galvanized, copper, pvc or pex.
(2) A leaking sewer drain line, usually cast iron, galvanized, clay or ABS.
(3) Naturally high groundwater table.
(4) Over watering of plants around your home (creating a man made high water table).
(5) Sprinklers spraying through the crawlspace vents.
(6) Leaking swimming pools (this is rare).
(7) Improperly directed gutter downspouts.
(8) Stream or river flooding.
Standing water or extremely damp soil in your home's crawlspace (under house) can cause a variety of structural problems and health concerns. Excess humidity caused by the evaporation of standing water or wet soil in your crawlspace can cause mold growth on the wooden foundation and the bottom of sub flooring. It can also cause warping or buckling of sub flooring and mold growth in the living area of the building. If your home has H.V.A.C. (heating , ventilation, air conditioning) ducting in the crawlspace and the system is compromised with excess moisture, microbial amplification may result. All of these issues can possibly create unhealthy indoor air quality. A broken sewer line in your crawlspace and the subsequent release of raw sewage onto your crawlspace soil can create an awful odor problem in your home. Also there are possible health concerns to the constant inhalation of sewer gas from an open or broken sewer line.
Some solutions to the problems mentioned above. Try to determine the source of the water intrusion and correct it yourself. This may be as simple as redirecting sprinkler heads, fixing leaky outdoor faucets or redirecting gutter downspouts. If you suspect plumbing problems, you should call a licensed plumber.
Often, there is too much water under your house for a plumber to properly inspect the pipes, you may need to call a Water Damage Restoration company. Many water damage companies offer crawlspace de-flooding and or sewage cleanup, should the leak be from a sewer drain line. After the water is removed and the crawlspace made accessible a plumber can check for leaky pipes. If your foundation is wet you may need a water damage restoration company to dry it for you, in order to prevent mold growth.
If you have over watering problems or a high water table you may need to contact a landscape engineer or a water proofing contractor to design a French drain system or other remedy to keep water from accumulating under your home. Many plumbing companies or water proofing contractors will also install sump pump systems in the low area of your crawlspace to help combat water pooling. You may also want to consider having a crawlspace dehumidifier installed to regulate the relative humidity to a level that will inhibit structural damage or mold growth. Another way to keep air flowing through the crawlspace (and thereby promoting evaporation) is by installing crawlspace ventilation fans over your existing crawlspace vents. There are also companies who can completely encapsulate your crawlspace with thick plastic, thereby effectively sealing your home off from the ground.
As you can see there are many problems that can arise from excessive water or moisture in your home or buildings crawlspace and just as many solutions. Should you suspect a problem that may need a professional solution, a good place to start is on the internet with your favorite search engine and a search for "crawlspace". This should get you started in the right direction with any crawlspace (under house) issues you may have.