Brisbane, Australia – Grass was extremely sponge, first time low mow and scarrifer revealed this. Is it rotting?

Hi all, this patch was exactly like the visible green. My foot in a large boot would sink down to the ankle at minimum. Finally mowed low as instructed before using a scarrifer for the first time and this is what's underneath. I did another patch elsewhere in the yard and it lacks this distinct black look underneath. Is this rotting? I struggle to effectively mow this every time due to how spongy it is, but the top layer always looks so healthy. Any and all tips appreciated. Thank you