Tafsir of At-Takweer 81:6

It means they are heated, in that their waters recede and fire appears in their place. Therefore, it has been said that the sea is the cover of Hell, or that they are filled by the flooding of some into others until the salty and the fresh become one single sea, derived from the phrase "I stoked (sajara) the oven" when one fills it with firewood to heat it.

It is also said: they are filled with blazing fires to punish the people of the Fire.

It is also said: they are filled with earth to level them with the ground of the Gathering place, though this has no basis in the reports of the predecessors.

It is reported in Lughat al-Qur’an (The Languages of the Qur’an) regarding "the sea" that sujjirat means "gathered" in the dialect of Khath‘am; perhaps their gathering [is intended] by way of their overflowing.

Ibn ‘Atiyyah said: It is possible that the meaning is that they are restrained and their surging is curbed so that they do not overflow from the earth out of terror. This is derived from the sajur of a dog, which is a piece of wood placed on its neck; one says "I sajarahu" when one binds him with it.

Ibn Kathir and Abu ‘Amr recited sujjirat with a light [un-doubled] letter (sujirat).