In papers submitted to the planning department in Palo Alto, Calif., Zuckerberg says that he wants to build four new houses to replace the four he recently purchased for tens of millions of dollars. One of those houses, however, will feature rather interesting fixtures that sound more like a panic room than your standard house.

One home, according to the plans, will feature “white brick walls, dark steel doors and windows, dark grey siding and louvers where they occur above the roof line, and a dark gray standing seam metal roof.” That house stands in stark contrast to the other three homes Zuckerberg plans to build, which the plans say will be “a simple palette of painted wood shingle siding, natural cedar shake roofing, painted windows and french doors, and stained wood doors where they are solid.”

While the plans don’t say how Zuckerberg would use the homes, the one outlier sounds somewhat like a panic room—or perhaps more appropriately, a panic house—that could be used to protect Zuckerberg and his family in the event of a threat. Indeed, it’s not uncommon for rich and famous people to have safeguarded rooms in their homes to protect themselves against threatening individuals. The house Zuckerberg proposes sounds like it could be a bigger version of those rooms and could be used if he needs to get out of his main residence.

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The New York Post, which earlier reported on the plans, described the proposed home as a “bunker,” though the house wouldn’t be underground.

There is reason to believe that Zuckerberg is thinking about security with its proposal. He did, after all, start buying the houses around him in 2012 to ensure his neighbors couldn’t get an easy glimpse at him or his property. According to several reports, Zuckerberg was concerned that his neighbor was building a house that could violate his privacy. The purchases, which are believed to range anywhere from $30 million to $44 million, depending on the document source, were made at vastly higher prices than the homes’ worth. One house, according to reports, was valued at $3.71 million; Zuckerberg acquired the home for $14 million.

Zuckerberg’s security is one of Facebook’s (FB) chief concerns as well. A recent regulatory filing shows that the company paid $5 million last year on security for Zuckerberg, including a security detail, private planes, and more. The sum also includes “security measures” for Zuckerberg’s personal residence.

Having a place to go in the event of danger could be especially important for Zuckerberg, who in February, was indirectly targeted in a 25-minute video by a group calling itself the Sons of the Caliphate Army. To make matters worse, people in his position can be targeted, Timothy Horner, head of security risk management at Kroll, told Fortune in an interview last year, by everyone from a “crazy niece or nephew” to terrorist organizations.

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Once Zuckerberg gets the go-ahead to start building the properties, he plans to build all four at the same time, according to the papers. When complete, their square footage will be 20% less than the homes they’re replacing.

“Our goal is to ensure the homes and surrounding gardens blend seamlessly with the neighborhood and feel as if they have always been there,” the papers read.

Facebook referred Fortune to a project spokesperson who declined to comment on the report.