Considering the fact that in the scientific sense man was half blind before modern discoveries, the Greek had every right to make up stories about how the world was made. Mythology told tales that were heroic, moralistic, and sometimes all out odd about how the world came to be how it is. And although the idea of monsters beneath the earth that belch flames through holes in the ground thus creating volcanoes may be a silly idea now... I think your argument may have been otherwise then.
Long, long ago (a period which many Greek Myths are set in), there was Zeus1, king of gods, who controlled lightning and ruled high up on Mount Olympus. Zeus was known to not only have taken his sister Hera2 as his bride and the Queen of Gods, but to have several other separate affairs with humans in the meantime. His reason was simple, if there were more humans that were part-god, there would be better, more moralistic and wise people.
It was because of this that Hera kept a close watch for him on Earth from high on Mount Olympus. Even Zeus feared his wife's wrath. One day, while watching, Hera noticed an oddly placed cloud blocking her view of the Earth. She knew Zeus was fooling about with other women, and mortals at that!
She quickly arrived and discovered that he was alone with a cow. However, Hera saw through the disguise and knew that the cow was really a woman that Zeus had transformed into the form of a cow3! The woman was a girl named Io, and Hera was furious. But Hera played along and took the cow and tied it to a tree without admitting to Zeus that she knew who the cow was.
The tree the cow was tied to was closely guarded by Argus, a powerful warrior. Argus was known for having 100 eyes placed all over his body. He was mighty and had slain Echdina, the terrifying mother of monsters such as the Sphinx4, Minotaur5, Nemean Lion6, Cerberus7, Chimera8, Ladon9, and Hydra10. Argus guarded the tree well, and Io stayed prisoner there for a long time.
Zeus, terribly sorry for this, devised a plan with the messenger god, Hermes11, to free her. Hermes came to Argus to entertain him for a bit. Argus, bored with his job, naturally paid attention too Hermes as he did all sorts of things.
But Hermes then started into the longest, dullest, most boring story that ever existed or ever will exist12. The dull story started to make Argus, the great and powerful 100-eyed warrior, actually fall asleep. One by one, every eye on his entire body shut until the two eyes in the natural area finally went down. Hermes cast a spell on the sleeping Argus, an eternal sleep. Argus had truly died of boredom.
As Io was released, Hera sent a troop of bees after her. Running, she went from Greece to Asia Minor to Egypt13! After this long trip, the bees were stopped, and Io became worshipped in Egypt for being a mysterious white calf.
Zeus made an agreement with Hera; Io could be a human as long as Zeus never even laid eyes upon her again. Io became a human, and several mortal relatives of Zeus were born to Io14.
An issue rose up afterwards though. Hermes was in a trial amongst the gods, for he was proclaimed by Hera to have murdered Argus. This could be agreed upon, but Hermes had an argument so persuasive that the gods simply could not go against him. The gods, major and minor, each had stones with their names written upon them. They were asked to cast the stones upon either Hera's feet if the believed Hermes guilty or upon Hermes feet if the found him innocent. Hermes' argument caused him to be buried in stones by the end of the trial, for only a few gods tossed them at Hera's feet15.
As for Argus, to preserve his memory, Hera took his eyes and placed them upon a bird that was later known as the peacock's tail. Ever since, peacocks had spots all over their tail feathers.