I just noticed something that is very troubling. We live in Northern California in the Sierra Nevada foothills. We have a 20-year-old pool built by a leading local company. The pool was here when we bought the house ten years ago.
The pool is on a slope. Our ground here is like iron in the summer and it's August. Yet the shallow end of the pool appears to have settled recently so that it is about an inch lower than the deep end. Before, the waterline was even with the tile line all around the tiled portion of the pool. So it is easy to tell if the pool is not level. The length of the pool runs parallel with the slope (meaning the shallow end and deep end are both set in about the same slope).
The redwood deck around the shallow end has settled about 1-1/2 inches below the edge of the pool but that happened slowly over many years, and the deck structure is on a separate foundation from the pool. The concrete slab that butts into the deck cracked 5-6 years ago but the crack has never widened. The change in the water level is very recent; I look at it often and didn't notice it until today.
You can see the side of the pool from under the deck. There is a concrete buttress in one area below the skimmer. The ground has eroded around its base and there is moisture at the base and water is dripping slowly from the concrete at the bottom. This is the only sign of a leak I can find.
We had a big hidden leak in the waterfall bowl opposite the buttress on the uphill side that I repaired a few weeks ago. We don't use the falls much and I didn't realize it was leaking until I left the falls on overnight and the pool was down 3" of water by the morning. I think the waterfall leak (about 3,000 gallons) may have undercut the pool and caused one end to settle. I never found one drop of that 3,000 gallons--it just disappeared into the ground under the pool, apparently. There are no visible cracks or bubbles in the pool wall anywhere except as noted with the spa, below, and no other sign of water besides the wet buttress at the base of the pool wall on the downhill side. I think the buttress water may be coming from a small leak in the skimmer above it; I once saw a few tiny bubbles coming out of the skimmer but didn't see any today even though there is fresh water on the buttress.
For the last year or two I have noticed that the top two rows of tiles around the spa on the pool side are not solidly attached to the wall. Just a few weeks ago one popped out and I pulled three more out before they fell. The spa wall has had a vertical hairline crack for several years that does seem to have gotten very slightly worse lately. There is about a 1/8" crack across the piece of slate that forms the spillway bed. This crack is in line with the crack in the spa wall below. Before there was just a crack with no gap in the slate.
Please see attached pictures.
20,000 gallon free-form custom pool. Plaster lining. Attached 1100 gallon spa. 2 skimmers, deep-end and side drains. Polaris 280. IntelliFlow pump w/ Compool controller and Pentair DE filter. Pentair heater, copper-and-aluminum solar heating.