I don’t know whether you would call this a “substance,” but the jet from an explosive shaped-charge will burn through a lot of armor in milliseconds.
And back on the subject, for a good layman-accessible rundown on the dangers of flourine see Death In The Laboratory by Isaac Asimov.
bizzwire
Yes there can be negative pH values. Speaking just off the cuff, I believe 1 molar Hydrochloric Acid is 0 pH. 10 molar HCl is -1 pH.
I feel somewhat neglected here as on this thread I’ve already posted a link to Iron Mountain which has -3.6 pH levels and no one seems to have noticed. 
Ah, well…
–Patch
My sense of humor.
Poor patchbunny. Well, I read your post and liked that quote. It seems like a description for some type of FPS or a movie. Can you imagine working in a cave with rocks and acid constantly falling around you? Not to mention the giant rocks MADE of acid. I hope those guys are being well paid.
Actually, that might be kinda fun. I wonder if there are heaps of gold at the bottom.
Scientists have actually invented the strongest possible acid several times, but they can never patent it because…
They don’t have anything strong enough to keep it in!
(old joke)