... balloon rigid. I’m sure emergency deflation would require venting most, of not alll of the helium and air. I m sure there is no way of compressing it. Imagine the size and weight of the cylinders! I seem to recall from my visit that the cost of the helium to fill it is about £250,000. They reckoned to replace about 10% each year. 😢 I guess they may be covered by insurance.