Contains spoilers for One-Punch Man chapter #170
One-Punch Man just reminded readers that the biggest threat to the strongest hero Saitama has yet to make its debut, despite being teased a long time ago. While some believed that Garou, the Hero Hunter who became the most powerful monster thanks to the influence of the nefarious God, was the "greatest catastrophe" prophesied by the seer Shibabawa, the manga revealed that this great calamity is still waiting to happen, and it could very well be Saitama's first real challenge.
Shibabawa was a famous fortune-teller with the power to predict disasters with 100% accuracy, down to the exact timing. She was held in great esteem by One-Punch Man's Hero Association, which used her talents to predict particularly dangerous attacks from Mysterious Beings (aka monsters). In chapter #28 of the manga, while predicting the future for the next half year, Shibabawa foresees the arrival of the greatest catastrophe of the era, which could be the end of Earth. She then dies, apparently from the shock (actually, she just choked on some cough drops), so she is unable to divulge the details of the catastrophe. Since then, every time that the planet has been in danger, some believed it to be Shibabawa's prophecy, but it has now been revealed that the Hero Association knows more about it than what previously believed.
Chapter #170 of One-Punch Man deals with the aftermath of the epic battle between the Hero Association and the Monster Association first, and then between Saitama and Garou. The Monster Association was already considered to be the biggest threat that the heroes ever faced, and its leader Orochi could only be beaten (in secret) by Saitama. However, Garou proved to be an even bigger danger, especially after receiving God's power and transforming into Cosmic Garou. Now that the dust has settled and Garou has been beaten and brought back to his senses, Sitch, the Minister Officer of Justice of the Hero Association, considers accepting him after his rehabilitation, despite the backlash it could cause. The reason is that strengthening the Association in preparation for the coming of the great prophecy is more important than anything else.
During the attack of the alien warlord Boros and his Dark Matter Thieves that completely destroyed one of the largest cities on Earth, Sitch believed that was Shibabawa's prophecy coming true, but then he changed his mind, seeing how easily the heroes (actually Saitama) defeated the invaders. Even One-Punch Man's powerful Monster Association, which went very close to eradicating the heroes, was not considered by Sitch the subject of Shibabawa's prophecy. Sitch's musings in chapter #170 confirm that this half-forgotten plot thread is actually very important for the story. In all likeness, the real "great catastrophe" will be something so dangerous that even Saitama will have trouble dealing with it. According to recent developments in the manga, this could be God finally getting free from his dimensional prison in the Moon.
One fan theory proposes that the real "great catastrophe" is actually Saitama, whose monstrous power could pose a threat to Earth's survival. As recently seen in the manga, a simple sneeze from Saitama can destroy a planet that is 1,000 times the size of Earth, so if he ever lost control the bald hero would certainly be the greatest threat possible. It remains to be seen if the next arc of One-Punch Man will finally unveil the truth about Shibabawa's prophecy, and introduce Saitama's new, and perhaps strongest, enemy.