This dark line running across about 25 strings is corrosion – very likely from something spilling or dripping into the piano. Unfortunately, it’s an all-too-common sight. You should NEVER put beverages, flower pots, fish bowls, or any other liquid-containing items on top of a piano. Especially a grand piano!
Compare the tuning pins in these two pianos. They are around the same age and both are school pianos. I advised the music teacher that the one on top was very concerning because of the amount of corrosion. Rust on strings and tuning pins is always a red flag! Having another piano around of similar age and design helped demonstrate the problem in this case. Often piano owners don’t notice rust or corrosion in their piano because it looks “normal” to them or they don’t ever look inside their instrument. Sometimes you can learn a lot just by a quick look!
Besides the rust, there is another problem with this picture. The wire coiled around each tuning pin is flush against the plate. This happens after several decades of tuning and settling, and is largely unavoidable. Once the coil is touching the plate, tightening the tuning pins is difficult and the piano’s tuning stability can suffer. Eventually the tuning pins have to be replaced with a slightly larger size, at which point it usually is most sensible to simply re-string the entire instrument because of the labor involved. Fortunately, this piano was about to get re-strung anyway!
Water got inside this piano due to a home water leak in the ceiling above it. Notice the significant rust on the strings to the left, and mold and discoloration on the hammers to the right. Fortunately, in this case, homeowners insurance covered the cost of replacing the hammers and the strings in this piano, since the damage was not structural or pervasive and repair costs did not exceed the value of the instrument. Most homeowner’s or renter’s insurance policies will cover damage to pianos that is the result of water, smoke, fire, other types of home damage, or natural disasters. I’m happy to provide an estimate of repair costs for insurance purposes and will work with your insurance company to determine the best solution.