Why didn't Mark Watney start digging downward?
I've watched The Martian and I've read the book, yet I still don't understand one thing.
If he ran out of horizontal space for crops, why didn't Mark Watney start digging downward?
In the book he used part of the Hab's floor canvas to make a "mars tent" for sleeping and working (which was dropped completely from original release and present as pretty big 100% Earth-made tent in directors cut), so he could make a "hatch" without losing the Hab completely. Digging downward through soil, rocks, etc would be a problem at first, but Mark is creative, he could make some kind of pickaxe. In a few month he would have an underground area for planting more potatoes (plus in the book he had peas and corn, yet he chose potatoes only... for some reason), which would give him way better chances of survival till Ares 4 (and a stronger hit, when the Hab actually blew up).
So, why didn't he dig?
P.S.: I'm not asking this on scientific communities of Stack Exchange, because, frankly, considering Martian radiation, very weak sunlight and pretty bad (although not completely unusable) soil, the book/movie is not really a hard science... Not hard enough, at least.
Best Answer
Because he would need to make an airtight seal between the walls of the Hab and the ground.
What is shown in the movie is that the Hab is a prefabricated 'tent', and Mark shifts a significant amount of Martian rock dust into that space through the airlock and makes soil that can support a potato by adding "biological materials" and water.
In this scenario he doesn't have to make this airtight, it is already. To dig down into the ground he would have to make a reliable seal between the Hab materials and dusty rocky ground, which would be extremely hard.
I'm also not entirely convinced that the extra area that he might be able to generate would be easy or even sustainable. For this to be worthwhile he would have to dig down and make a cave that extends beyond the border of the Hab - with all the physical work removing the material out of the Hab via an airlock, which would be backbreaking. Then he'd have to provide enough lighting and water. He has a limited amount of hydrazine to make water - which would be absorbed not only into his new planting soil, but also (uselessly) into the walls and roof of his cave.
His air appears to be a relatively replenishable resource from the the 'airmaker'/oxygenator that the story mentions, which is presumably solar powered. The water is not infinitely replenishable, and every time he goes outside to get rid of a bag of rock from his mining he dumps 1 airlock full of water vapor.
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Answer 2
The question on the potatoes was directly addressed in the book. They gave the most calories per area of farm land.
Couple other points where you appear to have miss interpreted/remembered some details. The hab is not sealed to the ground. When they got there they laid out the floor materials, staked them down, attached the canvas for the sides and top, and hooked up the other components. It is a free standing tent with a semi rigid floor and lots of accessories to keep the people in it alive and comfortable.
When Mark made his bedroom/working tent, he took a portion of the canvas from the hab top & sides, and the floors of the emergency tents from the rovers, and remanufactured things to get a larger single tent in place of the two smaller ones. There were several references in the book to the Hab ceiling being lower on one side after this because of the missing canvas.
Also the hab was providing more than just atmosphere containment for the farm. It also had light and heat. I am pretty sure there was a line in the book about the hab using UV lights for the health of the crew, which made them good grow lights for the plants. Trying to replicate any of those conditions outside of the controlled environment of the hab would have been enormously more difficult, both in terms of technical feasibility, and the level of effort expended.
Keep in mind that his primary issue was calorie deficiency. He had plenty of vitamins and supplements to last for years if he could find enough calories to keep his body running. Anything that involved additional physical exertion, without directly and immediately contributing to his survivability, would have made the problem worse.
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