The massive AWS outage hurt 54 of the top 100 internet retailers — but not Amazon (AMZN)

Amazon Web Services has become so powerful that when it goes down, it takes down a large chunk of the internet with it, as the world found out Tuesday.

Systems were back to normal on Wednesday, according to the company and other sources that watch the internet. On Thursday Amazon offered a detailed explanation why its S3 cloud storage service went awry.

The incident highlighted one scary thought for internet businesses: that a single company, Amazon, and its technology are responsible for much of the revenue to be made in cyberspace.

During AWS' four-hour disruption, S&P 500 companies lost $150 million, according to analysis by Cyence, a startup that models the economic impact of cyberrisk. US financial services companies lost an estimate $160 million, the company estimates.

Specifically, the web-monitoring company Apica, which watches for internet-performance issues worldwide, crunched the numbers on how AWS' downtime affected retail sites. It discovered that 54 of the top 100 internet retailers were affected with a decrease of 20% or greater in performance, and three websites went down completely: Express, Lululemon, and One Kings Lane.

Websites that on average usually require a few seconds to load took more than 30 seconds.

Top websites' load times increased by triple digits, according to Apica:

"Yet Apple, Walmart, Newegg, Best Buy, Costco, and surprisingly Amazon/Zappos were not affected by the outage," an Apica representative told Business Insider.

Apple does use some AWS services, though it is widely known to use Google's cloud, too.

Amazon and its e-commerce sites like Zappos were most likely spared for an easy reason: They have designed their sites to spread themselves across multiple Amazon geographic zones, so if a problem crops up in one zone, it doesn't hurt them. This is, naturally, the recommended way to use a cloud service, even for those using only the 800-pound gorilla of the cloud market, Amazon.

But it's also more complicated and more expensive, things that most other e-commerce sites would prefer to avoid.

In the meantime, take a look at this chart from ThousandEyes, a company that monitors the internet for performance problems, which shows just how dead AWS S3 became for a time.

When that AWS flatlines like that, almost the whole internet is in trouble.

Amazon could not be reached for comment.