Complex sentence: Cueva de los Verdes is formed from parts of the volcanic soils and extends from 1 km to 6 kilometres of old lava streams, which are the second longest in the world.
Split into simpler sentences: Cueva de los Verdes is formed from parts of the volcanic soils and extends from 1 km to 6 kilometres of old lava streams. The streams are the second longest in the world.