What does the term "six feet under" mean? [Poem: "Conversation," pg. 123]

being in a lot of trouble
feeling lonely
feeling very small
being deceased and buried

deep in the ground

where darkness surrounds
a place where silence is sound
six feet under, peace is found

no longer burdened by pain
no more tears to stain
the soul is free to reign
six feet under, where truth remains

a quiet conversation with the earth
a final resting place of worth
where troubles no longer have a berth
six feet under, a new birth

so when life feels too much to bear
remember that there's solace there
six feet under, a peaceful lair
where all can find a moment to spare.

being in a lot of trouble

feeling lonely
feeling very small
being deceased and buried
pick one

being deceased and buried

Is being decreased and buried the correct answer?