Because all those energy sources are natural and will always be there, when ever there is a need for extra energy.
Answer:
There is no actual way that the mantle could be...'exposed' or 'seen'. But scientists have managed to find multiple ways to analyze the mantle and it's actual existence by using multiple devices. The mantle is basically a 2,900 kilometers (1,802 miles) thick blocks of rocks and minerals underneath the crust(or the surface of the earth) , and makes up a whopping 84% of Earth's total volume.
Cells require a structure that regulates the transfer of material in and out of the cell. This structure is the –
Cell membrane.
