Carboniferous

The Carboniferous was a period in Earth's history, lasting from 359.2 to 299 million years ago. Because of the vast amount of forests growing across the globe during this period, the atmosphere had much higher oxygen levels than the present, allowing gigantic species of insects to evolve and thrive.

Real life
The Carboniferous period lasted from approximately 359 to 299 million years ago. During the Carboniferous, the atmosphere's oxygen content was at 35%, which was 14% higher than today's oxygen levels and was the highest level the oxygen content ever reached. The land was covered with vast swamps and rainforests, and land-based life had become well-established. Amphibians were the dominant terrestrial vertebrates, and arthropods, some of which grew to very large sizes, were also highly common.