The declining fossil fuel energy bounty is a fundamental and hidden driver of decline and collapse. This needs to be much more widely understood.
Simply put, the useable energy we get from any fuel is somewhat less than what is extracted from the ground. We need energy to mine the fuel, to process it (especially in the case of oil), and to transport it. So there's a ratio between the quantity we mine, and the quantity we can actually use. This measure is known as EROI ie Energy Return on energy Invested).
As we use up the most readily accessible and readily processed fuels, we progress to seeking fuels that are harder to mine and harder to process. And as a result the ratio between the energy we mine and the energy we can actually use gets worse for us. At some point in the life of every mine or oil/ gas field the operator will decide that it can no longer sell the product at a price that covers its costs. Extraction will stop.
This 5 min video 
makes it clear.
But that's not the end of the story. So much of our economy works on the basis of bountiful energy. And as the quantity of useable energy declines this has a direct impact on our economy.
This academic paper from the Royal Society in 2014 demonstrates how this works: "I conclude that, as the EROI of the average barrel of oil declines, long-term economic growth will become harder to achieve and come at an increasingly higher financial, energetic and environmental cost." No doubt this paper was peer reviewed.
No need to read the whole paper - <b>just read the abstract!</b>
A question we might ask ourselves is "why isn't the understanding of this important mechanism widely shared?"