Minecraft – How to collect a large amount of sand/glass without ruining the natural environment

I've seen some good answers on this question so far, but there is a way to to light the underside of the platform without making holes that go all the way through by using artificial means like lava.

If the platform is at least 3 blocks high, you can hollow out the middle layer and fill it with lava, with a pattern of glass below like so:

XXXXXXXXXXXXXXX  X = Stone (or whatever)
XLLLLLLLLLLLLLX  L = Lava
X^XXX^XXX^XXX^X  ^ = Glass

Perhaps you could pattern the glass on the bottom layer to resemble flourescent lighting. If your building material is flammable, you can replace the lava with lightstone or torches.

I have done this many times while building a bedrock-layer minecart track to connect my various dwellings. The strategy I have used with great success is to temporarily block the lava flow with a scaffolding of nether brick fence. Being made of stone, it will not catch fire and burn up like regular wood fencing, but it will still allow you to reach through it to place your glass blocks behind it.

In addition, the spaces in and around the fencing can give you enough room to place the glass against the sides of either a fence post on the edge of the scaffolding or a previously-placed block of glass. This means you can still build your glass tunnel even if you can't reach a nearby wall, floor, or ceiling of the lava pool to place the block against. This is very handy if you hit a lava pool that is very wide and/or deep.

It should also be noted that these same techniques are useful if you hit an underground pool of water, although in that case regular flammable fence would do just fine too.