At this point oil stores are reaching maximum capacity, with any free space going for a premium. I think that the squeeze at the end of the month has shown to a lot of people that if you don't have storage room for oil, its a liability.
Good for you if you have a ton of empty storage though!
No bet has good/amazing returns if there's no risk. No matter how remote it is, to be safe you sacrifice some profit. When you don't protect yourself for those remote risks, _if_ they bite you, it hurts real bad.
I'm not sure how many people planned for oil to ever go under zero. I think that many people are going to be eyeing the current storage level (and consumption!) and deciding that the risk (and cost) of having to settle a shipment might not be worth it. But I don't really know ¯\_(ツ)_/¯
https://www.eia.gov/petroleum/storagecapacity/ mentions the actual capacity limits from last September.
It sounds like Cushing, Oklahoma, had 76,093 thousand barrels of total storage and 59,641 of them were used last week. But that's been increasing sharply and might be much closer to the total right now.
59,641 thousand barrels isn't the highest amount that's ever been stored there, but it's getting close:
https://www.eia.gov/dnav/pet/hist/LeafHandler.ashx?n=PET&s=W...
I don't know how to get more current data, but I just found this through some web searches, so I bet there's much more information available.
https://www.eia.gov/petroleum/supply/weekly/pdf/table1.pdf
But in terms of total capacity, it's tricky, because you can turn the tankers that currently transport oil into floating stores, convert water tanks, salt mines, and other storage media into petroleum storage, etc.
We're probably at about 60% capacity or so here in the US, we haven't even filled up the SPR yet.
Right now, nobody wants to buy the oil to actually use it.
So the only buyers are people who are willing to store it now, in order to sell it later when the price is better.
Those people are "pricing in" their costs to store the oil in what they're willing to pay for it on the spot market.
As storage becomes more expensive to procure (demand for it is rising, all the "cheap hotels" are sold out, etc.), the price for oil goes down further to cover those costs of storage for it.
Ultimately, the negative prices are a reflection that nobody wants that oil now, and the more negative, the more a reflection that nobody is going to want that oil for awhile.
So the price of oil at the moment is a function of time (how long no one will want the oil) and the available supply (and therefore price) of storage for the period of time when no one wants the oil.