There were a lot of bugs in the 1.7 update with item replication, and
most of those have been fixed in 1.7.2. However, there is one bug still
working (at the moment) that allows for easy mine shaft creation. This
will undoubtedly be fixed in the near future, so take advantage of it
now.



First, build a large tower out of dirt. You need to be high enough off of the  ground so that the explosions don't cause TNT blocks to ricochet back up to you, like what happened in my first try with this. Build a small 2  block wide platform that is six blocks long. On the left side, remove  the last 2 blocks.




Place a sticky piston in the hole facing outwards.




Attach a TNT block to the piston.




Place 2 lines of redstone one block apart from each other.




Place a redstone repeater facing towards the piston on the left side.


Place another repeater in the opposite direction.




Place a redstone torch next to one of the redstone wires to provide power to the circuit.  Remove the torch after placing it and the power will stay in the  current.




This is the tricky part. Break one of the redstone wires and immediately  replace it. This will make it so the current has a small portion of "off state" and "on state" which causes the piston to rapidly expand and  contract.




Place a redstone torch on the end of the last dirt block. This will trigger  the TNT when the piston is expanded, causing it to fall down and  explode. TNT will also stick to the sticky piston and not explode, so  each time it expands, a new live TNT block is created.




Sit back and watch the carnage.






When you've finished blowing stuff up, break any part of the redstone to stop it. Admire your handiwork from above