Mackenzie River flows into the Beaufort Sea across a delta front which is about 50 miles (80 kilometers) wide. During the dark, cold days of winter it is hard to see that a delta exists at all. The river is frozen and blends in with the flat coastal plain. But by springtime the ice has melted, revealing a fan-shaped network of rivers, streams, lakes, and islands. The layout is never the same; the sand and mud changing the course of channels,and building or eroding islands. The most recognizable features are the conical mounds, known as pingos. There are over a thousand of these dotted around the delta, the largest concentration in the world. At the center of each is a block of solid ice that pushes up the soil into a hillock. The mounds grow each year, but a number of them collapse in the spring when the heat melts their ice core—the center caves in, resulting in a high-sided pond. The oldest mound on record was 1,300 years old and reached a height of 160 feet (50 meters). The Mackenzie runs from the Great Slave Lake (North America’s deepest) to the sea, but its catchment area is the size of Europe.