What in the $%^& - undercoating?

[lks dcvn]

I was looking at a truck and (as always) - I asked for pix of the key locations...

He (the current owner) sent me pix of the underside of the cab...

I have never seen anything like this...anybody know what the heck this stuff is? I realize that it is some type of undercoating, but why...

I asked for pix of the floor pans on the inside - haven't seen those yet - - that should be interesting.

rockers are solid (replaced/rivets) - kick panels are solid (replaced/rivets)...windshield seam/upper metal area - no rust...

Inner passenger rocker - (from pictures below) seem to be gone...

I have NEVER seen this on the bottom of a cab - figured some of you others may have come across this...is this a red flag or should I wait to see the floor pan pix?

Thanks again for your assistance everybody!