A gas slug is a conglomerate of high pressure gas bubbles that forms within certain volcanoes, the agitation of which is a key driving factor in Strombolian eruptions. They start out as small bubbles of gas inside of volcanic magma. These accumulate into one large bubble, which starts to rise through the lava plume. Although the effect of gas slugs in lava is well understood, how they form is not well understood, but recent research suggests that they can form as deep as 3 km (2 mi) under the surface.