Note: The toilet is not flushing mercury into the sewer system, just a giant bucket.
This is a video of Cody from Youtube channel Cody's Lab trying to flush increasingly larger quantities of mercury down a toilet. He starts with just a couple drops, and builds up to an entire toilet tank full of the stuff -- 152-pounds worth. The mercury in the toilet tank is so heavy the flush handle is too weak to lift the flapper so he just reaches in there with his bare hand to do it. Also: the shot at 8:40 is what I imagine the Silver Surfer's ass looks like after a bout of food poisoning. I remember a plumber once told me if you want to test the plumbing in a house you should try to flush a whole package of hotdogs down a toilet. I have caused so many floods during open houses.
Keep going for the video, but feel free to skip around. Full-tank mercury flush at 7:55. Man, I loved how when the handle doesn't work "I'll have to think of something" immediately turns to "I'll just stick my hand in there".
Thanks to Matt L, who agrees this guy should try molten lava next.