The Natural Philosopher wrote:

The more I look at the average urban and suburban sprawl the more I feel
that actually what we should be doing is putting the roads deep down,
the houses just under the surface with light pipes and the gardens and
parks on top...Hobbiton? possibly :-)

What do you think are the pros and cons of this from a cost/benefit
environmental and general living feeling?

You would certainly save on heating costs as the ground temp a couple of
metres down is pretty constant. In fact with all the latest gov. ideas
in underground thermal storage, it may not need any at all
john2