Imagine a giant asteroid strikes the Earth a few years from now, blocking out the Sun and collapsing agriculture worldwide. We see it coming, but all attempts to redirect its trajectory fail.

At first glance, our chances don’t look good. The planet is engulfed in flames. Dead fish carpet the rivers and canals. Farmers lose most of their livestock. After just a few days, the air begins to cool, and global average temperatures plummet. Crops fail catastrophically, and the food supply system as we know it falls apart.

Yet what if I told you we were able to survive – that we managed to build a new food system by repurposing heavy-duty infrastructure and excavating knowledge from the past?

In addition to sheltering humans, subterranean spaces could be used to grow nutritious foods. Despite the lack of light, and the damp, poorly circulating air, certain crops can thrive here with the right approach. Thankfully, small-scale experiments in subsurface urban farming are already underway. For example, Paris harbours six sq km (2.3 sq miles) of untapped space in the form of redundant car parks, a portion of which the company Cycloponics has turned into mushroom farms.

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Currently less than 2% of our calories come from the ocean. Only 22% of all seaworthy vessels are used for fishing. When the time comes, aircraft carriers, container ships, tug boats and yachts should be commandeered for aquaculture, using sea lanes devised for an entirely different purpose.