Cafe1 wrote:
Here's the problem:

I have a 30" by 30" square concrete slab (underground pillar) in my yard
that is several feet (or more) deep. It used to be the base for an old ham
radio antenna tower that neighbors say was 40 or 50 feet high. The concrete
base was put in by the former owners who are now deceased. It sticks up
about 1 inch above ground level and I want to get rid of it.
snip
So, here's one goofy idea I was thinking about. If I made a little ridge
around the slab and poured some type of acid on it, would the acid eat away
at and/or dissolve the concrete? Anyone ever tried anything like this? Any
particular acid I should try if I am crazy enough to do this?

Yes, this will work.
However, it will require an enormous amount of acid.
This might not be very expensive, as barrels of conc acid are not very
expensive.
However, it's not a safe way to do it.

A big jackhammer will take care of it.
Combining this with a diamond concrete saw, to slice the top into a grid
pattern beforehand may work.