Regenerates his body from a head and becomes stronger than ever thanks to his Saiyan genes

"You're wrong, Cell! It ends NOWWWW!" With that, the Super Saiyan suddenly flooded his kamehameha with all his remaining power. The massive surge of energy caused the beam to expand rapidly, the bulge rushing toward the epicenter with a sense of purpose and finality.

"NO, NO, NO!" Cell screamed, seeing the sphere inching closer and closer to his position. "NOOOOO!" he cried in terror voice as the entire blast swallowed him up. The ripping, searing heat of Goku's ki tore into him, biting and tearing. He felt his flesh beginning to melt away, his arms and legs being shredded into atoms from the overwhelming power. His body was coming apart.

"AAAAAHHHH!" The sound of Cell's scream warped, then cut off suddenly as the blast ripped his head from his body. The beam pushed its way further out over the ocean until Goku was satisfied. It was done. Over.

- Bringer of Death Chapter 48

"So," he began slowly. The look in Goku's eyes had changed: now it was serious; the games were over. "You've somehow gotten a lot stronger in the past couple of hours."

"Ah, so you did notice," Cell said, smiling with a visible satisfaction on his face. "Indeed, it seems a side effect of my recovering from severe injuries is that I receive a boost in power. I believe it is the result of my Saiyan DNA...for which I have you and Vegeta to thank."

"Fine, you come back stronger from each battle the way we do." Goku's voice was uncharacteristically dark, his brow angled down harshly. "Makes enough sense, I suppose. But I don't understand how you survived in the first place."

...

Gohan spoke up. "But that blast destroyed you. You shouldn't be able to come back from nothingness."

"Ah, but that is where you have forgotten my greatest ability of all: ingenuity," Cell replied, his smile broadening even further. "True, Goku's attack was more powerful than mine, but that didn't stop me. I had the presence of mind to pull back and block just before the blast impacted. When my body was ripped apart—an event I only barely remember—the concussive explosion blew my head, or what parts of it remained anyway, away from the rest of my body."

"Damn it!" muttered Trunks to Gohan. "I knew something was wrong. Even if we blow him up a hundred times he'll just keep coming back! Isn't there any way we beat this freak?"

Cell continued his recollection. "When I regained consciousness, my body was slowly regenerating at the bottom of the ocean. Thanks to my Arcosian genes, I didn't drown; thanks to my Namekian and my Saiyan genes, I returned whole, with all my power and more!" He folded his arms, lifting his chin up in triumph, and chuckled. "I can never be defeated, Goku...because I can never be destroyed!"

- Bringer of Death Chapter 51