The Carboniferous period began 354 million years ago and ended 290 million years ago. It was the period in which much of the Earth's present reserves of hydrocarbon fuels were laid down. Life on Earth was different during the Carboniferous, with giant insects and massive spiders evolving in the oxygen-rich atmosphere. The Carboniferous enjoyed a mild climate, and much of the land surface is thought to have been swampy.
During the Carboniferous period, the Earth's continents were just beginning to come together to form the supercontinent Pangaea. This process put landmasses in close association with each other, producing large coastal shelf areas covered with shallow seas that were ideal environments for swamp-dwelling life. The sea level was generally lower than at present, as the poles maintained large permanent ice sheets. The oxygen content of the air was unusually high during the Carboniferous, up to 30 percent, which may have been a factor in the development of the enormous arthropods of the period.
The Carboniferous was the period in which reptiles differentiated from amphibians. The moon was somewhat closer during the Carboniferous, and it would have appeared somewhat larger in the sky. The Earth's rotation was faster as well, and days lasted less than 23 hours.