Complex sentence: During these meetings, the idea that would later be known as PACER would be developed--the explosion of 1 Mt hydrogen bombs in large underground caverns to generate steam that would be converted into electrical power.
Split into simpler sentences: During these meetings, the idea that would later be known as PACER would be developed. PACER envisioned the explosion of 1 Mt hydrogen bombs in large underground caverns to generate steam that would be converted into electrical power.