It is formed along the lava stream because the outer part of lava, when flowing down, cool down faster due to contact with ground and air while the inner part remains its liquidity (hot) hence keep flowing and formed an empty space (cave). There are two cavities (in the lower and upper part) which would fit an adult.
This is a natural cave called 'susu' (milk) because the water flowing out of it is white/yellowish similar to milk. The white yellowish water is bicarbonate water emerges on the surface and forms sintered carbonate deposit (travertin/CaCO3). Upper part of this cave is commonly used as a meditation place. Around Susu Lake there is hot springs complex which can be used for bath.
