A rainy month and water in the crawlspace:
It's was an unusually wet April where I live. I went into my crawlspace last week to change the filter on the A/C and found water. The crawlspace is surrounded by concrete walls. I should point out that the water is only in the lower half the crawlspace and 2-3 inches deep in low spots. It would need to rise at least another 6 inches to threaten the furnace. Also, I'm new to this and never in the past figured out if I had a sump pump or not.
A strange basin with no pump:
To make matters more difficult someone had installed plastic sheeting along the floor of the crawlspace (I'm told this is to reduce radon, but the house failed the radon test when we bought it so it didn't work). There are several things covered up by the sheeting.
One of these things appeared to be a cylindrical basin. I removed the sheeting around the basin and found what looks to be a sump basin. It's a metal cylinder perforated many times on all sides buried in gravel with smaller gravel at the bottom. However, there are no drainage pipes that I can see and no pump. It looks to be just the metal cylinder.
Water disappearing…magically:
Now here is where it gets weird. Last night when I removed the plastic sheeting water rushed into the cylinder (looks to be about a 5 gallon cylinder). However, there was so much water that even when the cylinder filled up there was a good 5-15 gallons of water sitting on top of it. I went to bed and it rained all night. Concerned that the water may have risen I went back into the crawlspace to check on it.
All of the water sitting on top of the cylinder the night before was gone and the basin was only about half full. There was still water in the crawlspace but only where it had pooled up on the plastic sheeting. How did the water go anywhere if there is no pump and no drainage pipes?
UPDATE: I had forgotten I posted this question, but for what it is worth, here is what I learned later. The water in the crawlspace was not coming from rain but from a leaky faucet (it was an external faucet leaking indoors when running). I'm not sure if the radon mitigation sheeting would have been able to handle normal seepage but, since I had an active radon mitigation system in the garage, I left the plastic sheeting torn off and the dry well exposed. Ever since there have been no problems with water in the crawlspace.
Best Answer
It's a drywell... water goes in and is absorbed into the soil. If you are in an area with sandy soil, this is usually ok. In heavy rain seasons, the water table rises as more and more water is absorbed into the ground. As things return to normal, the water table drops.
If you're interested, Google for "water budget" or "groundwater budget" and you should find some more detailed explanations.
I'd recommend monitoring the situation. If it seems damp often, start to do what you can to direct stormwater away from the house. Put extensions on gutters and regrade around the perimeter of the house.