Rille

Rille is a type of volcanic feature. They look like long, deep trenches in the ground. They are created when lava flows underground and then cools, forming cracks and channels. The walls of the trench sometimes become very steep and can be hundreds of feet deep, making them look like large canyons. The most famous example of a rille is on the moon, in a place called the Mare Tranquillitatis, or the Sea of Tranquility.