Subterranean building

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First time poster here, long time lurker.
Basically I own some shitty hilly junk land.
I wanna put some storage in it but it's all hill.
I'm semi inexperienced with diy so I'm probably being retarded here. But I want to put a storage shed in this land, in being the key word, I wanna build a small subterranean storage shed, maybe 6ft by 6ft into a hill. I understand the critiques towards underground building with moisture and maintenance etc but I don't plan to live there any time soon so as far as I can see from practical obstacles it's just difficulty, maintenance and maybe killing myself but I'm open to additions? I mainly want to build a braced box underground and was wondering whether I'd be better off using one large box, or multiple modular boxes connected to create the overall structure , my intuition and physics education are telling me modular and small boxes? In addition to this just any tips or criticisms on what I'm attempting to do would be appreciated. Also if anyone could point me in the way of some equations which would help me determine how much pressure would be on my structure or will I have to compose my own?
pic unrelated