If you like big things, the Triassic era might be for you. Not only were dinosaurs evolving towards world domination, but some of that occurred on a geological feature 15 times the size of any modern equivalent. A vast delta plain on the edge of the Northern Ocean covered more than 1 percent of the Earth's land mass at the time. A new study has explained how it became the biggest such plain we can find in Earth's history.

Delta plains may not attract the awe of mountain ranges or coral reefs, but they are important for planetary ecology. Regular flooding by silt-laden waters makes the place where meandering rivers meet the sea exceptionally fertile. The flourishing of plants and animals in these places today makes it easy to imagine many dinosaur species first evolved in these areas. More recently, their role as the cradles of agriculture and therefore civilization give us plenty of reasons to be interested in their development.

Dr Tore Grane Klausen of Norway's University of Bergen wanted to know how the plain that bordered the Triassic Boreal Ocean (TBO) grew to a phenomenal 165,000 square kilometers (64,000 square miles). Close to the size of Washington State and larger than England and Wales combined, it was fed by a river system that drained an area as large as the Amazon. In Geology, Klausen and co-authors identify two essential components to the plain's expanse: a long period of sea level stability and the flatness of the seabed into which its rivers drained.

Today, the TBO delta plain lies underwater, between the northern tip of Norway and the island of Svalbard (Spitsbergen). Even if it was above sea level, the area would be home to little more than polar bears, but during the Triassic its climate resembled modern southern Asia.

The area has been heavily drilled in the hope of finding oil, but it is far less well known than if it was on land. Consequently, the size estimate is a minimum, with its boundaries in some directions not established.

Modern delta plains are less than 10,000 years old. Whatever existed beforehand was submerged as sea levels rose. Until we started building dams on their rivers, many were expanding, but there are limits to how fast they can grow. The largest, at the mouth of the Amazon, is just 10,800 square kilometers (4,200 square miles). 

Other ancient delta plains have been found that exceed those of today, but none match the TBO plain in size.