1988 P Roosevelt Dime. Something Underneath The Clad Layer?

Your coin looks to match number 10 in the list of Dirty Dozen Damages. Even though there is no premium value to these, you might try to find a nice example of each.

1) discoloration - stains from a beverage, or environmental damage from being buried, heated, etc.
2) scrapes over much of the coin - damage from sliding on pavement, a parking lot coin
3) coin bent or edges not round - it has been smashed with a hammer
4) coin blank on all or most of one side - someone machined the surface away or sanded it down
5) mirrored lettering - a vise job, a coin squeezed against another in a vise
6) rough, pebbly surfaces - a coin eroded by acid, it might now be underweight
7) smooth rims, smaller diameter - was trapped rolling inside a dryer, a " Dryer Coin", or tapped with a spoon
8) clear mounds on coin - glue that has dried transparently
9) small indentations in the shape of the letter D - marks left by the impact of the reeded edge of another coin
10) large blisters - coin exposed to high heat, such as in a campfire
11) shapes, often letters or numbers, not indented or raised - Pareidolia (like animal shapes in a cloud)
12) a circular scrape just inside the rim - " Ring of Death" caused by a coin wrapping or vending machine

Don't despair! Error coins remain ready to find from circulation, but they are outnumbered by unusual looking coins that merely have been damaged. If you can imagine a way to change an undamaged coin into one like you see, that's probably exactly what happened to it. Changes to a coin after it leaves the mint's striking chamber are considered post mint damage, or PMD, and have no premium value.

Yes, I'm thinking heat damage.

yes, heat damage noted.
We have been seeing may heat damaged coins here lately - is it the hot summer?