378:1
With stocks if you want to short a stock you have to have 50% of the value of that stock in the account. So if I had $20k I could short $40k worth of stock. But if the price of the stock goes up to $50k I have to add another $5k to the account. That is called a margin call.
But silver, shockingly is not like that. People who buy and sell silver don't actually want to hold it. You have to pay money to the shipper, and then you need a bank vault, security, etc. Because of this banks felt safe in selling "paper" silver. Meaning they were buying and selling ounces of silver on paper. At first the ratio was higher than with stocks but still based on a reasonable assumption that most people will not want to take possession of the silver.
Now there are several important reasons why it is in the interest of people to keep the price of silver low. If silver is low in price then the electronic equipment that uses it will also be lower in price. Currently the biggest consumer of physical silver is the solar panel industry. The second reason is that silver historically is used as a currency and can compete with the US dollar. For banks that simply create US dollars by writing debt they want to keep the dollar strong and the corollary of that is to keep silver weak. Think how easy it is, the price of silver is not set by people actually buying and selling physical silver, rather it is by buying and selling paper silver. The paper market for silver is much, much bigger than the physical market. So then, every time the price of silver goes up they simply short more of it on paper and keep the price low.
This is like a poker game, every time your opponent raises you respond and call that bid. Now the pot is huge, we have 378 ounces of paper silver for every ounce of silver in the vault. All it will take is 1% of the people who own paper silver to ask for delivery to cause a huge run on the bank. The only reason they would do that is because of a war.
But there is another factor with silver and that is the price of gold. The two are very similar precious metals. You can compare the cost of mining an ounce of gold with an ounce of silver. Historically the ratio is 10:1, meaning an ounce of gold has been worth ten times what an ounce of silver is worth. However, right now it is 87:1. So gold going up in price is causing silver to also go up in price.
Now imagine you own the short positions on the silver. Every time silver rises in price your liability rises.
Another way to look at this is we are playing musical chairs with silver, there are 378 people playing and one seat left. Two things will happen when the music stops, the price of silver will skyrocket and the bankruptcies from the ETF's that can't pay off those who are selling will also skyrocket. This market is about $14 billion. However, when silver skyrockets the market will be worth $20 billion when these ETF's go bankrupt.
But it is far worse than that, if silver skyrockets to $50 or $100 an ounce that means the US dollar has plummeted by 100% or 200%. You are talking about massive inflation.