What is this thing I saw on top of a hill when I went hiking?

Question

I live in Ventura county California. I went hiking up on a big hill behind my house. At the top there's a chainlink fence with barbed wire (an area about 100x200 ft). Rising a foot off the ground within the fence is a roof to some underground structure. It was caved in on one end so I looked in. It was like and empty warehouse with a dirt floor supported by beams. On one side of the hill was a large PBC pipe with water coming out of it that drained down the hill. Only I have no idea where the water was coming from since this thing was empty. This is one of many similar structures I've seen while hiking around town. The others were not as deep and had radio antenas near them.

Answer 1

Might be a marijuana plantation

Answer 2

It could be power plant, it's cooperative plant before that is being used , in order that there will be power supply or electricity.the pipe is being use in order that water will flow down the pipe in order to produce electricity

Answer 3

It is an old cistern used for fire fighting. They would store water in there to fight fires.

Answer 4

probably on old mine idk