This negative price is caused by greedy countries.
The usual worldwide oil demand is about 100 million barrels a day. Now, because of the Corona crisis, demand is about 30 percent down, so, ideally, the world oil production should be lowered with about 30 million barrels per day. So far, however, worldwide production only got 10 percent lower.
The BIG question is, who (which country) should reduce production? The biggest producer is Saudi Arabia, and they indeed reduced production, and called upon other producers to do the same, in the OPEC meetings. They (SA) don't have that much pain, SA is filthy rich, and can afford to keep the oil underground for future production, when prices are up again.
But how about other countries? Most countries do fun things with their oil dollars, like stimulate their economies. When they reduce production, they get into an economic crisis. Also, they produce smaller amounts than SA, so they figure, yes, we agree that production should go down, but no, we keep producing the same for a little while longer to not get into a financial crisis too soon. Or, because meanwhile oil prices are already much lower because of lower demand due to Corona, they even increase production, to make up for those lower prices.
This is called Greed. You don't care about the common cause of keeping oil price up for the benefit of your OPEC friends. You don't care that oil is already a finite resource and you don't lower production to save some oil for your grandchildren. You don't care of the air getting much cleaner and getting a little bit less global warming. No, you keep pumping like crazy for just a few lousy dollars.
The rest is history. You can't go on forever pumping 20 million barrels more than demand. Storage, meaning above ground storage, is very finite, and got full. Nobody will buy your oil, even at zero price.
The obvious solution is to store surplus oil underground, where it sat at no cost for hundreds of million years.
Arguments that production could not be stopped due to pressure buildup or so are moot, as the oil prices are down for many weeks now, and there was plenty time to calmly reduce production.
You may argue that the negative price could have been prevented to reduce production just one day earlier, because indeed you cannot close the valve in five minutes. But that ignores the fact that the negative price is just a result of a downwards trend ignored for weeks. Any downward trend will eventually cross the zero. Producers should have reduced production at the first signs of lower world oil prices, weeks ago, and argued that oil is a precious resource that you can only produce and consume once in 300 million years, and should be produced only in times of high prices. Producing precious oil for (positive) low prices is just scandalous, to say the least.
Update with a technical non-political aspect:
Today in the news, 9 hours ago:
Before the coast of California, for over a week 35 fully loaded oil tankers are waiting to be unloaded. They carry about 20 million barrels of crude oil, corresponding to about 20 percent of worldwide dayly pre-corona oil demand.
Imagine the cost of storing oil this way. If you could empty those ships now, you could earn some serious money.
If this was about mineral water imported from Europe (Vitel, Spa), the ships could just dump to the sea. But this is oil...