>but could pull someone up if super urgent This is the key, IMHO. We've simulated such things as human isolation and closed-loop ecologies before, numerous tim

>but could pull someone up if super urgent This is the key, IMHO. We've simulated such things as human isolation and closed-loop ecologies before, numerous times. And we did learn some things. But I don't think there is much you can do, and surely not anything ethical, that would correctly simulate being months away from Earth. Just as you said, when you're on-planet, anything that goes wrong can be dealt with. Have a leak you can't fix? Someone just had a medial issue and is unresponsive? You just can't take it any more? Whatever the case, help is just a press of the red button away. I'm not sure if you've played many video games on Hardcore mode, but it's a totally different experience to know your saved game file(s) are in jeopardy if what you're about to do goes south. I would imagine the feeling is somewhat like this, but worse. A lunar base might simulate it. An undersea base (as you suggested) might work too if you put it deep enough. But again, you're entering unethical territory here by making something exceedingly dangerous just for the sake of increasing the risk/stress.