Morlock and Eloi, I'm not sure becoming either of those appeal to me. I can see a lot of issues with underground living, there is the air circulation, the air cleaning and exchanging, and if being underground is because of bad air on the surface, well there is a lot of work to be done in the arena of cleaning/replenishing the air. Waste management also becomes a really big issue.
I am not sure but I think as far as a group of people trying to live in a 100% Self Sustained environment, nothing coming in or going out is not that long of a period. If I remember right the biosphere 2 experiment did not go to well. Many may point out the space station, but it has stuff coming and going on a regular basis.
We have a lot to still learn about the physical needs of ourselves and of our environment. I think a more practical solution than going underground would be domes that ride in the oceans kind of like in some movies.
An interesting example of Morlock and Eloi!
Yeah, I would definitely not live in an underground colony. But I was talking about underground cities that had free movement with the surface world. So, it wouldn't be like two secluded groups of population but kind of like how different cities exist in the same country. But yeah, living underground seems like a drag!