I recently have been working to finish my basement - the home is 30 years old and as I stripped the old floor off and exposed the concrete, I noticed that one corner of the floor sounded hollow for about a 4' x 10' area. As I was drilling holes to put tapcon screws into plywood, I noticed there was a distinct mildew or 'old water' smell coming from the holes closest to the same corner.

Is it possible the material under the concrete foundation has been washed away? If so, because if is in a corner this would mean that the concrete wall is sitting over a hollow area. This makes me VERY worried as, over time, this corner of the house may 'droop' and/or pull the rest of the house toward it over time.

On a positive note - there are NO cracks in the floor and I recently replaced a patio door on the main floor near this corner and did not find any major levelling issues.

I should note, too, that there used to be a downspout that ran underground at this corner of the house - it supposedly ran out the side of the house and down the front yard but all the evidence I see suggests the water ran under the house - which again suggests it washed out some of the material underneath the foundation.

SO, long story I know, but what are your thoughts on this? Should I push th panic button and start bringing in skipjacking crews and concrete experts or is the consensus that because I have limited the water now running through that corner of the house, I should be able to go several years without a problem? I plan on being in the home for 3 - 5 more years.

Perhaps relevant, I do have a swimming pool in the back yard that has had a leak near that area before as well - this has been fixed as well.