That's a big hole! All dug, or was TNT used too?
Also, I now don't feel like I've used too much lava, if anything I've been quite skimpy in its use!
The hole was all dug. I think I used about 5 or 6 TNTs which really didn't make much impact. If you look in the hole at the stair/pyramid shape the first screenshots are under that so removing 1 layer of blocks will open the whole thing right up.
I did get bored of the hole and started playing around with other stuff in between digs. Had 9 buckets of lava at a time to do all that and only killed myself once :)