[endoblast]

The temperature a few metres below ground level is consistently cool (approx. 15 celcius) year round.

Could this be made the basis of an efficient cooling system?

[hippo22]

It's not as simple as it might seem at first glance. People often go into their basement and think "wow, it's cool down here. If only I could make my house this cool." But, as soon as you moved the air from your basement to your house, the air in your basement would be replaced by ambient air and would take time to be cooled by the Earth. And so you quickly realize you need a lot of thermal mass and an efficient way to move heat in order to keep up with removing the heat from your house.

[7952]

Yes, but it needs to work both ways. Heat needs to be extracted during the winter. Otherwise the ground would just be heated up to much. That is what a ground source heat pump does.

[hylaride]

It's not so easy in dense urban environments where power cables are buried, along with ancient sewer systems, subways/metros, etc.

You are starting to see a lot more external AC (heat pump?) units jerry-rigged into the sides of multi-unit dwellings, though.