STACK Milan (Siziano) data center, Siziano, Lombardy

STACK Infrastructure's data center campus in Siziano, near Milan, Italy, has been operating since 2016 and now has three buildings with more than 50 megawatts of power live, after the third building, the 10-megawatt MIL02, opened in July 2022. STACK also holds expansion land at the site for more than 80 megawatts of total power. In April 2025 STACK agreed to sell its European colocation business, including its Milan data centers, to funds managed by Apollo, and it has not said publicly which of its Milan buildings, including MIL02, were part of that sale.

The campus began as Supernap Italia, a venture between the US company Switch and the ACDC Holdings fund; STACK says it chose the Siziano site in 2015 for its access to highways and railways, low seismic risk and room for electrical substations. IPI Partners bought Supernap Italia in March 2021 and rebranded it under the STACK name a year later; the second building opened in November 2021, bringing the campus to 40 megawatts.

Under the Apollo deal, Sherif Rizkalla, chief executive of STACK's EMEA colocation business, will lead a new, independent company for the sold data centers, one that will not carry the STACK name or logo. STACK, owned by Blue Owl Digital since October 2024, said it would keep operating in Europe with a focus on hyperscale clients, and it still runs multiple campuses in Milan. DCD reported in April 2025 that it had asked STACK which of its European data centers, including any in Siziano, were part of the sale; no answer had been made public as of that report.