Instagram, Facebook and WhatsApp have all gone down in a simultaneous and strange outage.

The three apps might appear to be run separately, even if they are all owned by the same company. On the face of it, there is nothing obvious to link the three different services, and each of them can be used individually.

However, the scale of this outage – and of similar problems in the past – simply highlights the shared infrastructure that powers much of the web. While the internet can appear as a process of visiting a variety of different places that seem not to be linked together at all, they are in fact held together by mostly invisible processes that mean they can simultaneously break.

Perhaps the most spectacular example of that was the previous outage of Instagram, Facebook and WhatsApp in March. That lasted for nearly an entire day, and was probably the biggest service failure in the history of the internet.

“By duration, this is by far the largest outage we have seen since the launch of Downdetector in 2012,” Tom Sanders, co-founder of Downdetector told Techcrunch at the time.

“Our systems processed about 7.5 million problem reports from end users over the course of this incident. Never before have we such a large scale outage.”

That last time, no explanation was immediately obvious for the problems. Eventually, Facebook admitted to the issue and gave an explanation – though it was still vague, failing to explain both what happened and how the three different apps can be so closely tied together.

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“Yesterday, as a result of a server configuration change, many people had trouble accessing our apps and services,” Facebook posted on its Twitter after the problem was fixed.

“We’ve now resolved the issues and our systems are recovering. We’re very sorry for the inconvenience and appreciate everyone’s patience.”

Presumably, since Facebook bought Instagram and WhatsApp, the servers and processes powering the three have been gradually tied together. That means that despite being ostensibly separate, they likely rely on the same infrastructure – and a problem with it can affect all three.

In the future, that is likely to become yet more prevalent. Facebook boss Mark Zuckerberg has committed to bring the three messaging apps together – allowing people to speak between Instagram and WhatsApp, for instance – and that shared backend will bring even more reliance on each other, and potential for simultaneous outages.

But the problem is not necessarily a consequence of the three companies being tied together by ownership. The nature of the web is such that – as the name suggests – each part is strung to every other, and even separate parts of the internet actually rely on each other.

In 2016, cyber criminals launched an attack on a company called Dyn. While that firm is largely unknown, its impact was huge – the cyber attack broke large parts of the internet, leaving many people and websites unable to get online.

The impact was so vast because while many people might not have heard of Dyn, they have almost certainly used its services. Dyn provides internet services that work as the very infrastructure of the web.

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In that case, the problem was with domain name servers or DNS. They work as the signposts of the internet: when someone types an address into their browser, it ensures their computer goes to the right place and gets the right information.

When that stops working, then, computers and servers start to get lost and users can’t see the websites they’re trying to look at. As in that case, such a problem can immediately cause vast problems, the cause of which might not be immediately obvious – taking down large parts of apparently unconnected parts of the internet, without a cause being immediately obvious.