Can people live in a mine, unknown to the rest of the world, and survive on only mushrooms?
It is not possible for the people to survive only on water. They would suffer from a deficiency of important nutrients. The people in the mine would also require water which is very essential.
No, you would need water, and vitamins not found in most mines.
No. You need more than one type of food, to get all te nutrients, an you would suffer severe vitamin D deficiency.