Where did the mud inside come from if it was still sealed?

limestone is porous and will allow water to eventually seep through.

a condensation cycle will occur, and drip percolate the soft tissue and adipocere into a slurry [coffin liquor] that will settle to the bottom of the sarcophagus.

I honestly cannot believe that I have been listening to Death Metal my entire life, and no one has ever used the term “coffin liquor” in a song.