Instead of building up ie:tower blocks etc. why dont they build down?

If they went down far enough they could use the Earth's natural heat to generate electricity. And it'd never get cold so no more heating bills ! No shortage of water either. You can pipe sunlight down through commercially available optical light-pipes, the only problem would be food, but presumeably given enough space you could grow crops underground as well. The only immediate problems I can see are lack of fresh air, and how to ensure the whole (no pun intended) lot doesn't cave in on top of you. But even that could be overcome with a little thought!

I'm all for it, I'm off to build myself an underground mansion !

Instead of building up ie:tower blocks etc. why dont they build down?internet browser

i dont know. it would be well funky...but then again there wouldnt b much air going down there. tough london undergrounds are quite airy..well when the train is coming in

Instead of building up ie:tower blocks etc. why dont they build down?windows media center internet explorer

Where would they put all the dirt -- tunneling is expensive.

People like to look out of windows. The big boss likes to be on top of all his employees.

It is hard to keep a large underground structure waterproof.
Of course because of the cost. The space above us are free, so we don't have to do anything with that. On the other hand, building underground requires far more effort and budget as we have to remove the soils before starting to build anything. Plus, building underground faces much more issues like worker safety during construction or the strength of the ground around the place to avoid collapse.
They do, but no one is supposed to know!
Sunlight is essential for good health; certainly the production of vitamin D, which is very necessary for a good skeletal system, and probably other things as well.
Because building down is the same thing

as mining. And if you ever tried it, you'll

notice pretty quick, that's it more explosives,

than building.
They do both.

Bramble
No Windows

It's expensive.

You'll encounter the water table = waterproofing required.

Difficult to ventilate

Waste water would need to be pumped upwards, as the sewers etc would be higher than most of the levels.

Most underground space in big cities is already used and would need extensive re-routing of cables, gas pipes, etc etc.

It's just not a product anyone is really going to want!
The deeper the hole you dig in the gound, the more expensive it becomes. Building a tower is generally cheaper.

What would happen in a flood?

How would you get enough light in?
Thats a very interesting question you asked but looking at it in the logical sense its really not appropriate.
Have you seen the weather out there?

Would you live in a pond?
A great idea; but it has two drawbacks that I can see: first, no natural light or ventilation, so more energy use; second, liability to flooding.
Lots of good points already:

sunlight

water proofing

air flow

One big one I'd like to point out is soil pressure. If we assume that a soil cube one metre cubed weighs 18kg (an approx estimate applicable to most soils), then at a depth of 1m you'd have a pressure of 18kg/m2 on the wall. At a depth of 10m you'll have a pressure of 180kg/m2. Keep going deeper, and you need thicker and thicker concrete to resist the pressure from the soil. Once you include that you have wet soil below the water table it gets worse. So building deep is very expensive.