According to Norse mythology the world will have a time of great reckoning and cataclysmic events, starting with the melting of Hel.
Hel (in Norse mythology) is a barren hostile wasteland of ice where only the dead live. It is essentially the Arctic. According to the myth of Ragnarok the North Pole / Hel will melt, the world will become flooded, and all the peoples of the earth will be forced to move farther inland. (In Christianity we tend to think of Hell as being an underworld place and describe it as hot instead, but in Norse myths Hel is an actual place and very cold instead.)
This flooding will be but the first of a series of cataclysmic events and "very treacherous weather" which will cause mankind to fight against each other over land and resources.
The myth of Ragnarok also says the Norse gods (Odin, Thor, Freyr, Heimdall and Loki) will all be killed in the cataclysm. Odin is basically the Norse version of Zeus, Jupiter, Jehovah, Allah or Shiva. The other gods have corresponding deities in other myths and religions too (including Christian angels / demons). The names may be different, but they're essentially the same gods with a Norse flavour.
After Ragnarok is over, the flood waters will retreat and the surviving humans will repopulate the world's fertile regions.
The word Ragna means gods and Rok means both origin and ending, so Ragnarok essentially means a time of godly destruction and rebirth.
Ragnarok is said to begin with three roosters crowing, the crimson rooster Fjalar (which means liar), the golden rooster Gullinkambi who lives in Valhalla and the unnamed soot-red rooster in Hel. Obviously like all prophecies these are meant as metaphors.
Then Garmr (the immortal dog) will howl and be set free, and all the gods will begin to fight in a great religious war.
Then Fimbulvinter (a very long winter that lasts for three years) and many wars will follow, including an invasion of giants. The fire giants end up setting fire to the earth and there is a huge slaughter as almost everyone is killed.
Norse mythology is rather unusual compared to other religious myths because it actually foretells the death of the gods.
Remember years ago how a lot of people were fussing about 2012 and the changing to the Mayan calendar, with some people thinking it signaled the end of the world. Well it was not the end of the world. Its just a cyclic calendar. In comparison Ragnarok is a much more interesting prophecy and has direct ties to what is currently going on with global warming, climate change and rising sea levels.
