I really love the ideas incorporated with this piece. The overall tone fits with the text perfectly and I love how each phrase pulls to the next. Very well done!
I only have a couple of things I'd like to point out, just personal preferences really.
I love the chords and all of the dissonance, but at times, --particularly in m37, I'm concerned that it might be too difficult for the remaining voices parts to enter on an 8-part clash. Granted, that comes with the performance difficulty you are aiming for, but I feel that an adjustment in that measure alone may bring the entire difficulty of the piece down to a more intermediate/ High School level.
The other issue comes from words. I love this text, but the repetition of the word "Atlantis" from m15-30 may lose some of the energy of the phrase. Musically, it seems fine, I would suggest making the chanting/whispering to include more of the text than just "Atlantis"
The piece reminds me a lot of Cloudburst by Eric Whitacre, especially with the layers that are created during whispers.
Very moving work, I would love to sing this!