Complex sentence: The immense 381 m high zero-pressure balloon is constructed from 5,000 kg of polyethylene, using helium as the lift gas at its maximum altitude the balloon's volume will be 1.25 million m³ (40 million ft³).
Split into simpler sentences: The immense 381 m high zero-pressure balloon is constructed from 5,000 kg of polyethylene, using helium as the lift gas. At its maximum altitude the balloon's volume will be 1.25 million m³ (40 million ft³).