Ar: Jahannama.
One of the most important theme in the Qur'an, perhaps an indication of the preoccupation in Muhammad's mind is that of imagery of hell and the punishment for the unbelievers. This is indicated by the huge number of Qur'anic references:
The world population has been growing exponentially, let's just say that today's population is about 1/5 of all people who ever lived (1/2 or 1/3 would probably be more realistic). Thus, out of thirty billion people, only 30 million make it to paradise, that is about 2.5% of all Muslims living today! If we consider this percentage in terms of today's population only, and assuming that only Muslims will make it, then we are looking at a maximum of 6 million Muslims entering paradise as opposed to 1.2 billion Muslims living worldwide!
Or expressed differently, something Muslims do not tell in their DA'WAH: everyone converting to Islam today has a chance of about 1 / 200 to go to paradise. And as the percentage of Muslims in the total world population rises, the probability for each individual Muslim to make it to paradise is diminishing even further.
Narrated Alhassan bin Sewar, narrated Laith (Ibn Saad), from Maawia, from Rashed bin Saad, from Abdelrahman bin Qotada Alsalmi that he said: I heard the prophet of Allah saying: "Allah created Adam, then he took the creation (the people) from his (Adam's) back , and said: These into paradise and I do not care, and these into the fire (hell) and I do not care."
For a detailed discussion, see the article The Majority in Hell are Women.