"Mined" is the past tense of mine: Archeologists have found
evidence that iron ore was mined in the Andes 2000 years ago.
"Mound" can be a noun: The check was buried under a mound of
papers on the table. "Mound can also be a verb: Mound the frosting
on the cake and use a spatula to spread it out and down the
sides.
Note that where "mound" is used as a verb, the past tense is
"mounded": He mounded the earth around the base of the newly
planted tree.