Complex sentence: The Olmec heartland is an area on the south coast of the Gulf of Mexico coastal plain of southern Veracruz and Tabasco about 125 miles long and 50 miles wide (200 by 80 km), with the Coatzalcoalcos River system running through the middle.
Split into simpler sentences: The Olmec heartland is an area on the southern Gulf of Mexico coastal plain, in Veracruz and Tabasco. It is about 125 miles long and 50 miles wide (200 by 80 km), with the Coatzalcoalcos River system running through the middle.