Help! How do I clean this...?

[cantcleanwontclean]

I have not cleaned the toilet 'properly' in... well, ever - around the underside of the 'rim' is now black / brown sort of grime that seems to have sort of... 'attacked' the ceramic in some way? Like I've tried wiping it but it is sort of part of the toilet now... (?). I've attached some pictures. What am I supposed to do about this? Can it be removed or has it in fact ruined the toilet? The rest of the toilet is passable as I wipe the structure of it and take a brush to the bowl regularly to ensure no unacceptable substances remain in the bowl. But this underside of the rim thing has been neglected unfortunately.
Help me please! I am clueless when it comes to cleaning.
Thank you

P.S. the bowl is slightly green streaked due to bleach residue when I took the pictures.

Toilet cleaner with limescale removing properties, scouring pad, gloves. Enjoy.

Or an actual real pumice stone. Marvellous.

Scrape with a wooden chopstick. It'll come off in satisfying scales.

Oh Lord , try rubber gloves bleach and some good old elbow grease?

Mr Muscle toilet cleaner and Limescale remover .
Use gloves and make sure you don't get it on your clothes, as it will bleach your clothes.
I know.

Acid. Vinegar for a start.

Watching out for any pipework, diluted 'Spirits of Salts' (that's conc. Hydrochloric Acid to most people) is your next bet.

That stuff is lethal, btw, so if you have kids, pets or vulnerable people around the house, then vinegar (distilled malt or the cheap Aldi non-brewed condiment) or the limescale toilet cleaners will shift most of it.

And elbow grease. Lots of scrubbing....

We inherited toilets far worse and I used up all my elbow grease on the minging cooker, used black bottled Harpic every night and a cistern block and they cleaned up a treat.