Background and description of deck now:
It measures 10' by 20'. Typical wood floor and railings with steps leading to yard. The top step area of the old cement steps leading out the back door is still there and decking is level with it.

Flooring is attached to welded angle iron which was laid out in grid fashion, angle iron is in turn welded to metal poles which are cemented into the ground at approx. 2 feet deep. There are a total of 17 of the metal poles in the ground. The deck flooring is about 30 high from the ground, give or take a few inches as dirt isn't exactly level underneath.

My problem with the deck:
Although it is built on the north side of the house, it gets too much sun and is way too hot to enjoy in the summer.
We have also put in a garden pond (still not quite finished) and as you can see, we removed 2 sections of railing where the chairs are for viewing.

What I would like to do:
I want to lower the deck until it is 12" from the ground. There are several reasons for this, the 2 main reason are, 1) House will help create extra shade. 2) I want to build a large pergola out from the house, over the pond area.

Possible Impossible solution? What would you do?
As the deck was built over 10 years ago, I hate to have to remove all the boards, because they may or may not go back in place. (But may have too.)
I had thought of supporting the deck with a bunch of jacks, and cutting out part of the metal pipes the exact same width, and slowly lowering it to the next piece and rewelding into place. The problem with that is I don't know of anyone small enough to crawl into a 12" high space and welding without catching the deck on fire.

So what can I do? I would love to have the simplest solution, which may be obvious, but what would you do and how?

Pictures:

Thank you.