Building a deep sea underwater city is likely more outrageous than an underground city on mars. You're not avoiding tail risks by doing something preposterously

Building a deep sea underwater city is likely more outrageous than an underground city on mars. You're not avoiding tail risks by doing something preposterously dangerous like this because you will almost certainly die regardless of the above world conditions. You'd have to deal with: * needs to be anchored to something somehow * no sunlight * likely catastrophic vulnerability to any tectonic movement * no easily accessible soil * extremely high, extremely slow costs of expansion * no clear way to build the initial structure. * need for perfect protection from the salt * heating problems * likely complete dependence on the above world for materials and food. mostly just going to be nuclear subs with a handful of additional disadvantages.