There they find countless sinners cased in red-hot coffins sunk in burning marl.

On questioning his guide, Dante learns each open sepulchre contains an arch-heretic, or leader of some religious sect, and that each tomb is heated to a degree corresponding to the extent of the harm done by its occupant's teachings.
_Canto X._ Gingerly treading between burning tombs and fortress wall, Virgil conducts Dante to an open sepulchre, where lies the Ghibelline leader Farinata.

Partly rising out of his glowing tomb, this warrior informs Dante that the Guelfs--twice driven out of Florence--have returned thither.

At that moment another victim, peering over the edge of his coffin, anxiously begs for news of his son Guido, thus proving that, while these unfortunates know both past and future, the present remains a mystery to them.

Too amazed at first to speak, Dante mentions Guido in the past tense, whereupon the unhappy father, rashly inferring his son is dead, plunges back into his sepulchre with a desperate cry.