Pillow lava is a type of volcanic rock that forms when lava erupts underwater. As the hot lava comes into contact with cold water, it cools rapidly, creating rounded, pillow-shaped structures. These formations can vary in size and are often found in oceanic environments, such as mid-ocean ridges. The unique shape of pillow lava is a result of the lava's outer layer solidifying quickly while the inside remains molten. This process can create a series of interconnected pillows, contributing to the geological features of the ocean floor and providing insights into underwater volcanic activity.