According to the article, it's not the completed tunnels, but rather, the dig sites. It's mostly normal digging byproduducts (silt, rock dust, sand, water), but also chemicals for making/setting grout. All of that stuff is "normal", but the outrageously lax safety standards have let it build up rather than be dealt with.
"Elon Musk Tunnels Oozing with Skin-Burning Chemical Sludge" is a sensationalist clickbait take. "Elon Musk's Tunnels-In-Progress Full of Improperly Stored/Disposed-Of Dangerous Chemical Waste" wouldn't spark the imagination so hard.
No, it's not supposed to make it better. It's worse. "His tunnels are oozing with sludge" is mysterious, meant to draw readers in. The truth is, like every goddamn thing he's ever done, he's mismanaged it so hard and cut so many corners that he's caused yet another entirely predictable, entirely preventable industrial disaster.
I can hate Elon Musk and clickbait at the same time, mate.
Alright fair enough, it came across to me like trying to make a bad thing sound reasonable by making it sound mundane which is a common tactic of his supporters.
Sorry, did you think that was meant to be a defense of the man? Do you think his fanatics would ever describe him as "outrageously lax"? Read, mate, read.
People here see a chance to try to crucify somebody they perceive as an imperfect leftist to show they're the one pure socialist and that takes precedence over reading comprehension.