Complex sentence: First announced in the early 1950's, the envisioned structure would have seated 52,000 people and been the first domed stadium in the world, opening roughly a decade before Houston's Astrodome.
Split into simpler sentences: First announced in the early 1950's, the envisioned structure would have seated 52,000 people and been the first domed stadium in the world. Had it been built, it would have opened roughly a decade before Houston's Astrodome.