Praise be to Allah.

What has been stated by the scholars is that Paradise is in the seventh heaven and Hell is in the lowest level of the earth. 

Al-Bukhaari (7432) narrated from Abu Hurayrah that the Prophet (blessings and peace of Allah be upon him) said: “If you ask of Allah, then ask Him for al-Firdaws, for it is the middle of Paradise and the highest part of Paradise, and above it is the Throne of the Most Gracious.” 

Al-Bayhaqi narrated in al-Ba‘th wa’n-Nushoor (455), with a da‘eef isnaad from Ibn Mas‘ood that he said: Paradise is in the seventh, highest heaven and Hell is in the seventh, lowest earth. Then he recited the verses (interpretation of the meaning): “Nay! Verily, the Record (writing of the deeds) of Al-Abrar (the pious who fear Allah and avoid evil), is (preserved) in ‘Illiyyeen” [al-Mutaffifeen 83:18] and “Nay! Truly, the Record (writing of the deeds) of the Fujjar (disbelievers, sinners, evil-doers and wicked) is (preserved) in Sijjeen” [al-Mutaffifeen 83:7]. Al-Bayhaqi said immediately after that: The hadith of al-Bara’ ibn ‘Aazib and Abu Hurayrah about the punishment of the grave, and what they narrated from the Prophet (blessings and peace of Allah be upon him) about the location of the soul of the believer and of the disbeliever are indicative of that. End quote. 

Al-Haakim (8698) narrated from ‘Abdullah ibn Salaam that Paradise is in the heaven and Hell is in the earth. Classed as saheeh by al-Haakim, and adh-Dhahabi agreed with him. 

Al-Haafiz Ibn Rajab (may Allah have mercy on him) said: 

‘Atiyyah narrated from Ibn ‘Abbaas, who said: Paradise is in the seventh heaven and Hell is in the seventh earth. This was narrated by Abu Nu‘aym. 

Ibn Mandah narrated from the hadith of Abu Yahya al-Qattaat from Mujaahid, who said: I said to Ibn ‘Abbaas: Where is Paradise? He said: Above seven heavens. I said: Where is Hell? He said: Beneath seven seas, one above the other. 

Ibn Abi’d-Dunya narrated with his isnaad from Qataadah who said: They used to say that Paradise was in the seventh heaven and that Hell was in the seventh earth. 

Some of them quoted as evidence for that the fact that Allah, may He be exalted, told us that the disbelievers will be shown to the Fire morning and evening, i.e., during the barzakh period [between death and the resurrection] and He told us that no gates of heaven will be opened to them. This indicates that Hell is in the earth.

End quote from at-Takhweef min an-Naar (p. 62-63). 

Ibn ‘Uthaymeen (may Allah have mercy on him) was asked: Where are Paradise and Hell? 

He replied: Paradise is in the highest of ‘Illiyyeen and Hell is in as-Sijjeen, and as-Sijjeen is in the lowest earth, as it says in the hadith: “When the person is dying, Allah, may He be exalted, says: ‘Record the book of My slave in Sijjeen in the lowest earth.’” As for Paradise, it is above, in the highest of ‘Illiyyeen. It is proven from the Prophet (blessings and peace of Allah be upon him) that “The Throne of the Lord, may He be glorified and exalted, is the roof of Jannat al-Firdaws.” 

End quote from Fataawa Noor ‘ala ad-Darb (4/2) 

The shaykh also said: 

The location of Hell is in the earth, but some of the scholars said that it is the oaceans, and others said that it is in the centre of the earth. What appears to be the case is that it is in the earth, but we do not know exactly where it is in the earth. 

The evidence that Hell is in the earth is the verse in which Allah, may He be exalted, says: (interpretation of the meaning): “Nay! Truly, the Record (writing of the deeds) of the Fujjar (disbelievers, sinners, evil-doers and wicked) is (preserved) in Sijjeen” [al-Mutaffifeen 83:7]. Sijjeen is the lowest earth, and Hell is in the earth. 

There are hadiths that have been narrated concerning this, but they are da‘eef (weak). There are also reports from the early generations, such as Ibn ‘Abbaas and Ibn Mas‘ood, and this is the apparent meaning of the Qur’an. Allah, may He be exalted, says (interpretation of the meaning): “Verily, those who belie Our Ayat (proofs, evidences, verses, lessons, signs, revelations, etc.) and treat them with arrogance, for them the gates of heaven will not be opened, and they will not enter Paradise until the camel goes through the eye of the needle (which is impossible)” [al-A‘raaf 7:40]. Those who disbelieve in the signs of Allah and are too arrogant to believe in them will undoubtedly be in Hell. 

End quote from ash-Sharh al-Mumti‘ (3/174-175) 

See also the answer to question no. 14526 

And Allah knows best.