There is no exact mention from the Quran or the sayings of Prophet Muhammad which pinpoint the location of Hell. No one knows its exact place except God. Due to some linguistic evidence and context of certain hadeeth, some scholars have stated that Hell is in the heavens, yet others say it is in the lower earth.
Hell is huge and immensely deep. We know this through several ways.
First, innumerable people will enter Hell, each, as described in a hadeeth, with the molar teeth as large as Uhud (a mountain in Medina) The distance between the shoulders of its inhabitants has also been described as being equivalent to three days of walking. Hell will house all the unbelievers and sinners from the beginning of time and there will still be room for more. God says:
"On the Day when We will say to Hell: ‘Are you filled?’ It will say, ‘Are there any more (to come)?’" (Quran 50:30)
The Fire of Hell is likened to a mill that grinds thousands and thousands of tons of grain and then waits for more to come.
Second, a stone thrown from the top of Hell will take a very long time to reach the bottom. One of the companions of the Prophet, may the mercy and blessings of God be upon him, describes how they were sitting with the Prophet and heard the sound of something falling. The Prophet inquired if they knew what it was. When they expressed their lack of knowledge, he said:
"That was a stone thrown into Hell seventy years ago and it is yet in the way (the other side) of Hell until now."
Another report states:
"If a stone as big as seven pregnant camels was thrown from the edge of Hell, it would fly through it for seventy years, and yet it would not reach the bottom."
Third, a great many angels will bring Hell on the Day of Resurrection. God speaks of it:
"And Hell will be brought near that Day…" (Quran 89:23)
The Prophet said:
"Hell will be brought forth that Day by means of seventy thousand ropes, each of which will be held by seventy thousand angels."
Fourth, another report that indicates the vast size of Hell is that the sun and moon will be rolled up in Hell on the Day of Resurrection.
Hell has various levels of heat and punishment, each is reserved according to the extent of their disbelief and sins of those being punished. God says:
"Surely, the hypocrites will be in the lowest depths (grade) of the Fire." (Quran 4:145)
The lower the level of Hell, the greater is the intensity of heat. Since the hypocrites will suffer the worst punishment, so will they be in the lowest part of Hell.
God refers to the levels of Hell in the Quran:
"For all will be (ranked) by degrees according to what they did." (Quran 6:132)
"Is one who seeks the good pleasure of God then like the one who draws on himself the wrath of God? His abode is Hell – and worst, indeed, is that destination! They are in varying grades with God, and God is All-Seer of what they do." (Quran 3:162-163)
The Gates of Hell
God speaks of seven Gates of Hell in the Quran:
"And surely, Hell is the promised place for them all. It has seven gates, for each of these gates is a class of sinners assigned." (Quran 15:43-44)
Each gate has an allotted share of the damned who will enter through it. Each will enter according to his deeds and assigned a level of Hell accordingly. When the unbelievers are brought to Hell, its gates will open, they will enter it, and stay in it forever:
"And those who disbelieved will be driven to Hell in groups until, when they reach it, its gates are opened and its keepers will say, ‘Did there not come to you messengers from yourselves, reciting to you the verses of your Lord and warning you of the meeting of this Day of yours?’ They will say, ‘Yes, but the word (meaning the decree) of punishment has come into effect upon the disbelievers.’" (Quran 39:71)
They will be told after admission:
"Enter the gates of Hell to abide eternally therein, and wretched is the residence of the arrogant." (Quran 39:72)
The gates will be shut and there will be no hope of escape as God says:
"But those who reject Our signs, they are the companions of the left hand. On them will be Fire vaulted over (meaning the gates will be locked)." (Quran 90:19-20)
Furthermore, God says in the Quran:
"Woe to every scorner and mocker, who collects wealth and (continuously) counts it. He thinks that his wealth will make him immortal. No! He will surely be thrown into the Crusher. And what can make you know what is the Crushers? It is the fire of God, (eternally) fueled, Which mounts directed at the hearts. Indeed, it (Hellfire) will be closed down upon them in extended columns." (Quran 104:1-9)
The gates of Hell are also closed before the Day of Judgment. The Prophet of Islam spoke of them closing in the month of Ramadan.
Stones and stubborn unbelievers make the fuel of Hell as God says:
"O you who have believed, protect yourselves and your families from a Fire whose fuel is people and stones..." (Quran 66:6)
"…then fear the Fire, whose fuel is men and stones, prepared for the disbelievers." (Quran 2:24)
Another source of fuel for Hell will be the pagan gods that were worshipped besides God:
"Indeed, you (disbelievers) and what you worship other than God are the firewood of Hell. You will be coming to (enter) it. Had these (false deities) been (actual) gods, they would not have come to it, but all are eternal therein." (Quran 21:98-99)
The Clothing of its Inhabitants
God tells us that the dress of the people of Hell will be garments of fire tailored for them:
"…But those who disbelieved will have cut out for them garments of fire. Poured upon their heads will be scalding water." (Quran 22:19)
"And you will see the criminals that Day bound together in shackles, their garments of liquid pitch (melted copper) and their faces covered by the Fire." (Quran 14:49-50)