An opening to the surface formed where a portion of a lava tube's ceiling has collapsed. If this occurs after the tube has drained, a breakdown pile will be evident beneath the skylight. But if the tube was still active, or had subsequent flows of lava through it, the breakdown will be carried off or buried.

Surface skylights can be clearly seen during the current eruption sequence from Kiluea Volcano on the Big Island of Hawaii. Viewed from the air, they appear as a string of glowing red pearls against the blackness of the pahoehoe surface. Scientists at the Hawaiian Volcano Observatory use aerial photographs to map out developing lava tube caves as they carry hot lava to the sea. The first photo taken by an Observatory scientists, shows one of these windows into an active lava tube.
The photo below was taken from beneath a skylight in Maui's Ke'alaku Caverns. The cave is thousands of years old, so the surface is lush with vegetation.