On April 28, 2025, the lights went out on the Iberian Peninsula. A widespread power outage paralyzed critical infrastructure in Spain and Portugal for approximately 23 hours. Media outlets across Europe reported on the blackout and the resulting Internet outages affecting large segments of urban infrastructure and the population. But core services remained active as data centers switched to emergency power supplies. However, a disruption in one fallback solution created a more severe impact on global data traffic than the blackout did.

Researchers at the University of Kassel, led by emergenCITY Principal Investigator Oliver Hohlfeld and emergenCITY doctoral researcher Yasin Alhamwy, examined these complex effects of the power outage, as well as the observed vulnerabilities and surprising resilience of the region’s central Internet infrastructure, in a detailed study. Together with researchers from DE CIX, they presented the first systematic analysis of the power outage on the Iberian Peninsula, examining exactly how the blackout affected the region’s Internet ecosystem. Their paper, “When the Lights Go Out: Internet Resilience During the 2025 Iberian Power Outage,” appeared in the April 2026 issue of the ACM SIGCOMM’s Computer Communication Review and has now been chosen as one of two „Best of CCR“ paper at ACM SIGCOMM in Denver.

Sudden Drop of Data Traffic to Nighttime Levels

For their analysis, the researchers combined measurements of data traffic at the four largest Iberian Internet exchange points with data from IP liveness scans and Border Gateway Protocol (BGP) updates. They cross-referenced the results with reports from the amateur radio platform PSKReporter. The result: When the power grid failed, data traffic at the region’s Internet exchange points under study dropped by 33 to 70 percent within a few minutes. This brought traffic down to levels typically seen during nighttime hours, indicating a widespread loss of end-user connectivity and IP availability.

However, the fact that data traffic did not come to a complete standstill shows that the central routing devices continued to operate on emergency power. By breaking down the remaining data traffic, the researchers were able to identify which services (e.g., content delivery networks (CDNs) and international transit) were the most resilient and which (e.g., enterprise and gaming services) nearly disappeared entirely.

“Understanding what remains online—and what goes dark—helps operators, policymakers, and the public prepare for future crises, whether they stem from natural disasters, grid failures, or other large-scale events,” emphasizes Oliver Hohlfeld.

Resilience of Backup Systems Crucial to the Resilience of the Global Internet

The Internet used by the public of the Iberian Peninsula thus proved to be extremely vulnerable to the blackout. At the same time, the continued operation of core infrastructure traffic—particularly transit traffic that neither originates from nor is destined to the affected region—underscores that services hosted in data centers are comparatively resilient.

However, the scientists’ observations also show that this resilience has its limits: A partial failure of the emergency generators in a large data center during the power outage led to a second measurable disruption of Internet traffic—remarkably, with a greater impact on the stability of global Internet routing than the one caused by the nationwide power outage itself. These findings point out that while data centers are fairly well protected against local outages, vulnerabilities in their backup systems can amplify the effects of incidents that are already critical.

Hohlfeld and the study team therefore draw two key conclusions from their analysis. First, the core Internet infrastructure of the affected region is more resilient than often assumed. However, according to the researchers’ second point, it is also important to keep an eye on the resilience of concentrated infrastructure such as data centers and to ensure they are independently audited. Understanding this interplay is crucial for improving preparedness and developing more robust systems to handle future disruptions of a similar magnitude.