He also launched a mission to convert the Tatars, but he died before it could start. To secure the necessary quiet for his medical studies, he had an apartment added to the papal palace at Viterbo, to which he could retire when he wished to work undisturbed. On 14 May 1277, while the pope was alone in this apartment, it collapsed; John was buried under the ruins and died on 20 May in consequence of the serious injuries he had received. He was buried in the Duomo di Viterbo, where his tomb can still be seen. After his death, it was rumored that John XXI had actually been a necromancer, a suspicion frequently directed towards the few scholars among medieval popes (see, e.g., Sylvester II). It was also said that his death had been an act of God, stopping him from completing a heretical treatise. Since the works of "Peter of Spain" continued to be studied and appreciated, however, Dante Alighieri placed "Pietro Spano" in his "Paradiso's" with the spirits of other great religious scholars.