Out of the blue.

Over the course of several weeks, a blue lake turned blood-red.

The shade-shifting Lake Voui (VOO-ee) sits atop a volcano on Vanuatu, an island in the South Pacific. Scientists say that magma deep inside the volcano is partly to blame for the sudden color change.

This molten rock contains hot water and dissolved elements such as iron. When the heated fluid rises to the volcano's surface, it pools at the bottom of the lake.

Typically, the searing fluid--including the dissolved iron--remains buried at the bottom of the lake, giving the water its blue hue. But strong winds or more vigorous Volcanic activity can stir up the lake, sending the deep water to the surface. There, the iron reacts with oxygen in the air. The elements combine to create a red form of iron oxide known as rust.

These fine iron oxide particles stay suspended in the lake, "so it will stay red for quite a while," says Johan Varekamp, a geologist at Wesleyan University in Connecticut.