The idea of how humans can achieve immortality to live for hundreds or thousands of years isn't possible. Using proper advanced technology, scientists were able to unlock the secret of long life. However, there's already evidence proving that the existence of immortality is possible due to petrification. Petrification is a process in which organic matter is exposed to a certain mineral over some time and then turns into stone. There is evidence such as fossils that are petrified and their body along with their organs are still intact.
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The idea of petrification is demonstrated in the anime Dr. Stone, there was a bright light flashed in the whole world that petrifies humanity and turned everyone into stone. After a thousand years, the main lead woke up due to a natural mineral (nitric acid) that frails the stone. The sense of petrification is quite possible to happen in real life. This might be the solution to turn humanity into stone to let the planet recover its vegetation for hundreds of years.
Petrifying humanity might be the best solution to solve every environmental dilemma that the world comes across. Without humans, the planet will heal itself, as time goes by man-made structures will collapse then trees and the whole ecosystem will recover bringing back the planet to the stone age. At present, it's impossible to petrify all humans in this world since there's no such technology capable of turning everyone into stone however there is an interesting lake on this planet which could turn anything into stone and that is Lake Natron.
Lake Natron can be found in Northern Tanzania and it was considered a deadly lake due to the presence of alkaline since it was located near a Volcano. Despite Lake's beauty, it was deadly to any living organism that drinks at the lake due to the presence of cyanobacteria, a type of algae that damages cells, nervous system, and organs of any animals who drink from the lake. The main reason why animals are petrified in the lake is due to the extreme content of soda and salt, those animals are calcified turning them into petrified animals.
The scientist might use the idea of calcification to identify the exact mineral or substance that petrified those animals. This can be useful to understand how petrification works, it sounds evil but the idea of petrification of humans isn't possible in the future to achieve near-immortal life and woke up someday seeing our planet full of vegetation.
The next possible way to live for hundreds is the most effective and scientifically proven experiment which is cryosleep. Cryosleep is a process in which humans are put in a deep sleep at a super low temperature, this keeps the metabolism low at very possible. This method is usually seen in movies related to interstellar travels like relocating to another planet which takes hundreds of years of journey in deep space.
The ship's crew is put on cryosleep and the system will automatically open those chambers once the ship's already reached the surface of that planet. This idea is quite possible in the future, once everything here on earth is devastated and it's time for the human race to relocate to another terrestrial home then everyone should be in cryosleep to maintain their body and physical slowly aging even if it's already a hundred years had passed.
With the help of cryosleep, humanity can achieve life for hundreds of years however it's still not that close to immortality since everyone is in deep sleep and only machine provides for the body's needs. This possibility isn't far to happen in the future, as technology advances, more scientist discovers ways toward evolution and new knowledge.
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Petrification
https://quotetheanime.com/dr-stone-petrification-theory/
Lake Natron
https://www.livescience.com/40135-photographer-rick-brandt-lake-natron.html
Cryosleep
https://www.quora.com/Is-cryosleep-possible-as-shown-in-Interstellar-2014-film