The volcano that the title of Under The Volcano makes reference to is Popocatépetl or El Popo or Smoking Mountain .For reference I think you say it like popo-catta-petal. The book also mentions the neighbouring Iztaccíhuatl (White Woman apparently) but the still active Popocatépetl is the main focus. The names and the believed origins of the Mountains come from Mexican folklore, The Warrior Popocatépetl and The Princess Iztaccíhuatl.
"As Aztec mythology has it, Popocateptl and Iztaccihuatl were once humans who were deeply in love. The legend is about two star-crossed lovers, the young brave warrior Popocatepetl and the princess Iztaccihuatl. Iztaccihuatl's father who was a mighty ruler, placed a demand on Popocatepetl before he could take Iztaccihuatl as his bride. He required that Popocatepetl first engage in a battle against the tribes enemy and return victorious. Some variations of this legend include the stipulation that Popocatepetl must return with the enemys' head as proof of his success. Story has it that Popocatepetl went off for war with.
Iztacclhuatl waiting for her beloveds' return. Popocatepetl won the battle and was ready to return to Iztacchuatle when word reached the ruler that the warrior had been slain. Upon hearing the false news, the princess, Iztaccihuatl falls ill and succumbs to her deep sorrow, dying of a broken heart. When Popocatepetl returns to his people with triumph, he encounters his beloveds' death and he is heartbroken and inconsolable.
Popocateptl carries Iztaccihuatl's body to the mountains where he has a funeral pyre built for his princess and himself. Grief stricken, he dies next to his beloved. The Gods, touched by the plight of the lovers, turns the humans into mountains so that they can finally be together. Today, they remain with Popocatepetl residing over his princess Iztacclhuatl. On occasion, Popo will spew ash, reminding everyone watching that he is always in attendance and that he will never leave the side of his beloved Izta."
The picture below are of Popocatépetl :)