Grotta del Cane Watercolour showing the Grotta del Cane at Pozzuoli, near Naples. This grotto contains a layer of carbon dioxide (a volcanic feature) reaching c 30 cm above ground level. In the 19th century this was illustrated to tourists by taking a dog into the cave. The dog would collapse, while its handler was unaffected. Accordingly, a man is shown removing an unconscious or dead dog from the grotto. A signature reads "Chez Glass Naples" early 19th century