A new type of lava cave is described from the summit crater of Ol Doinyo Lengai, a unique active carbonatite volcano in Tanzania. This and other similar caves on Ol Doinyo Lengai are formed by thermal erosion and aqueous dissolution of otherwise solid spatter cones. Meteoritic water and endogeneous condensates act to form speleothems of complex mineralogy up to 3 m in length. We propose the new classification of "polygenetic spatter cone cave".