In 1916, Karl Schwarzschild found a formula to calculate the size of a black hole, formally known as the event horizon. What not many people know is that when he tested his new formula, he found a theoretical limit to how large a black hole can be. He wrote that if a supermassive black hole would continue "eating" matter after reaching this limit, it would likely become unstable and collapse on itself. His calculations showed that this event would generate gravitational waves so large that our entire solar system would be torn apart, regardless of where in the universe the collapse occurred.
His results were dismissed by other scientists as "a science fiction tale" and they pointed out that such large black holes could never exist. Many of them were surprised when none other than Albert Einstein said he looked at Schwarzschild's results "with the utmost interest". Einstein was correct: after Schwarzschild's death, multiple black holes close to the limit were found. The currently largest known black hole, TON 618, is estimated to be at 99.7% of the limit and is still eating as we speak...