I found this on healthline.com: "Why is my pee dark yellow and sinks?"

"Dark urine is most commonly due to dehydration. However, it may be an indicator that excess, unusual, or potentially dangerous waste products are circulating in the body. For example, dark brown urine may indicate liver disease due to the presence of bile in the urine."

or this:

"What does it mean when my urine is so heavy, it immediately settles to the bottom of the toilet and leaves the rest of the water uncolored? There are several reasons why it might do so, but the simple explanation is that the urine is denser than the water and so it sinks.

Being dehydrated will make it denser, it’ll have more salt in it and it’ll sink. Having more protein in it will make it sink too, as could such things as a kidney or maybe bladder infection.

If it stops sinking like this when you drink more fluids, probably not a worry but if not, I think I’d see a doctor, if only to rule out possible infections."