Imam Sadeq (AS) was asked:

What is the grave? Imam (AS) replied: The grave is Barzakh. Imam (AS) was asked: What is Barzakh? Imam Sadeq (AS) replied: It is the same as the grave from the time of death until the Day of Judgment [1]. According to the holy Quran when disbelievers and sinners die, angels torture them in Barzakh, as mentioned in Ayah 50 of Surah Al-Anfal: “If only you (the Prophet (PBUH)) could see, when the angels take the souls of disbelievers, how they strike their faces and backs; it will be said: Taste the punishment of the fire.” Also in Ayah 27 of Surah Mohammad: “How will they feel when the angels take them in death and beat their faces and their backs.” Therefore, the pressure of the grave is the same as those pressures and torments inflicted in Barzakh on Barzakhi bodies by angels and not on the worldly bodies. Even some of the dead ones who do not have a grave will face the same situation in Barzakh.

[1] Beharul Anwaar, Vol. 6, Page 218.

Source:en.shafaqna.com