451. The group is known as Yajooj Majooj. 

Verses 18:94, 21:96 of the Quran talk about a group known as Yajooj Majooj. 

These people would expose themselves at the time when the whole world is to be destroyed. And are known as Yajooj Majooj. And they exposing themselves is one of the ten signs of signalling the nearing of doomsday was declared by Prophet Muhammad. These people have existed in this world for a long time, in an area surrounded by high mountains. This area surrounded by mountains with gaps in between them being filled up with iron blocks and sealed with molten copper making it impossible for them to exit that place until the doomsday, when the barrier will be broken, and their numbers will be very enormous making it difficult for them to exit the place say verses 18:94-96 of the Quran. And verse 21:96 of the Quran says they will finally have the barriers broken; they will come as a flood of water does from a higher elevation.

This verse also declares the opening of these to let them out of their confinement, would happen on the close heels of the doomsday.

........ Allah will send the group known as Yajooj Majooj, the first of whom will drink water from the water reservoir known as Tabriya, and they will continue to descend from all high places on earth, at a time when Prophet Isa and his followers will be encircled by them, and the siege would continue till a time when the value of the head of a cattle would be equal to a hundred cold coins of the present day, and Prophet Isa would plead with the Almighty, when immediately all belonging to the Yajooj Majooj would fall dead. Prophet Isa and his followers on being freed from the clutches of the Yajooj Majooj would come down to the plains from the Tor mountains only to encounter a dirty unbearable stench of human corpses strewn all over the place without any gap between them. At such a situation Prophet Isa would again plead with the Almighty to relieve the world from the stench, when birds with long necks the like of a camel would descend to clear all the corpses of men belonging to Yajooj Majooj of the earth. And Allah would send down torrents of rain that would wash the houses whether they be tent or mud houses on earth immaculately clean like a glass. Then Allah would command, Oh! Earth let the fruits grow again and spread prosperity. At that time a single pomegranate would be enough to satiate the hunger of a huge crowd, the skin of the fruit would be large enough to provide shelter to a number of people. There would be blessings of Allah on milk and milk from a single female camel would suffice the needs of a big group of people. And milk from a single cow would be sufficient enough to fulfill the needs of people of a clan. Milk from a single goat would be enough to feed the entire family. At such a juncture clear air would be sent from above by Allah that would capture the lives of all Muslims, leaving alone only a few, and this is the time when the whole world gets to be destroyed, as conveyed by Prophet Muhammad. Book Muslim: 5629. 5629 

The location where they are being held in isolation is not disclosed either by the Almighty or Prohet Muhammad. This may be due to the fact their identity by people is necessary only close to doomsday.

Some question the ‘isolation of Yajooj Majooj by asking a question, at a modern time like the present with so many gadgets available would it not be possible to locate them? Or at least identify the copper wall by its brightness? This is an inappropriate question.

Though man has invented many modern gadgets, they have not been used to their full extent. Though man has taken pictures of the earth from high above, he has not taken pictures of each and every nook and corner of the world. This can be understood by going through details about certain places on earth being discovered recently. 

There are numerous locations on earth, man has yet to set his feet. Though man can investigate hidden objects from high above with the help of modern gadgets, it is not possible for him to accurately pinpoint everything on earth because of the forest cover, and various other factors.

Forest and caves on earth cannot be penetrated with the help of telescopic devices. 

And if the Yajooj Majooj happen to be located under the canopy of forests or caves no modern gadgets can locate them. 

The copper metal soon gets deposited with all kinds of vegetation like the algae etc. and turns green losing its luster. Giving an appearance green in color, whether seen from a distance or close quarters. 

Hence the existence of Yajooj Majooj on this earth invisible to anyone cannot be viewed with skepticism.

In the future man can be expected to encounter the group and the time may happen to be close to the doomsday.