From the dawn of time, humans have wanted to know how to live forever. After all, the end is a frightening, unknown thing, and a chance to avoid it entirely is almost too enticing to pass up.
Nearly every culture throughout history has fantasized and written stories about what it might be like to skip the whole "involuntarily releasing your bowels as you die" trope. Whether they attempt to sell their soul to the devil or perform daily pilates (or both, simultaneously), everybody has their own way of staving off the Grim Reaper just a bit longer.
The bad news is many of the ways to become immortal come with pretty nasty side effects. But hey, if you think becoming a flesh-eating zombie is worth it for the chance to live for eternity, then maybe you should give one of these methods a try.
Although many Westerners think of mermaids as sexy women of the sea who are both half-fish and half-clothed, in Japan, mermaids look a little different. There, mermaids are called "ningyo," and these creatures are not so much Ariel as a monkey head and a carp body plus a beautiful voice. Fishers were warned to avoid catching them because doing so would bring bad luck.
The legend of the ningyo immortality, often referred to as the Yao Bikuni story, explains one day, a young Japanese girl happened upon some of that sweet monkey-fish mermaid meat. After eating it, the girl was magically granted immortality, and because of her inability to age, decided to become a Buddhist priestess. Living forever may sound attractive, but eating the flesh of a mermaid isn't worth it, as you'll have to watch everyone you know get old and eventually perish.
Although not exactly biblical canon, there has been a story circulating for approximately 800 years about a man who achieved immortality via an encounter with Jesus. Allegedly, while Jesus was struggling to carry the cross through the streets of Jerusalem before his crucifixion, a man came up to him and taunted him, telling him to go faster and striking him with something. Jesus, presumably annoyed and about to be crucified, told the man that because of his actions, he would have to "wait" until Jesus' "return." And by wait, Jesus meant sit around for 2,000 years and counting.
The man realized what Jesus meant and immediately converted to Christianity. He wasn't able to die, but his "gift" of immortality had some pretty unfortunate side effects. For example, every hundred years, he would get sick and go into some kind of fit before magically turning 30 again. In addition, in some versions of the story, he could only rest on Christmas, otherwise he was doomed to wander the world restlessly.
Becoming a vampire can either force you into a life of solitary reclusion where you're forced to avoid sunlight and drink blood or you might just become sparkly when it's sunny. Either way, most vampire legends assume vampires are immortal in that they will never age. Vampires can still be slain - whether by the aforementioned sunlight, not drinking enough blood, or getting staked in the heart - which is a pretty effective way to take out anybody.
The toughest part of becoming a vampire is finding a vampire willing to bite you and leave you alive. In the Twilight saga, new vampires are only created with much hesitation, and in other stories, vampires simply drink the blood of their marks until they've perished. One must be a strong negotiator to gain the immortality provided by vampirism.
The problem with angering an immortal deity is that they have the potential to hold a grudge for all of eternity. There are several instances in Greek mythology where a god would get mad at a mortal and punish them but would also grant that individual everlasting life. That way, their horrific punishment would literally never end.
One example of this was when Sisyphus, a normal mortal king, angered pretty much all of the gods by being something of a know-it-all. He would often attempt to trick or barter with gods instead of bowing to them, and eventually, they cursed him to live forever in Hades and tasked him with rolling a huge boulder up a hill. Every time he got to the top, the boulder would roll back down. Sisyphus is presumably still pushing that rock up a hill to this day.