Complex sentence: At about 1000 feet the sub imploded, hit bottom breaking off about 50 feet of the bow, then slid 2/3 mile down the side of an extinct volcano finally resting on a notch on that underwater mountain.
Split into simpler sentences: At about 1000 feet the sub would have imploded, then hit the bottom, breaking off about 50 feet of the bow. It then slid 2/3 mile down the side of an extinct volcano, finally coming to rest on a notch on the underwater mountain.