The Justice Department hand-delivered special counsel Robert Mueller’s 448-page report to Congress at about 11 a.m. Thursday—on CD-ROMs. Why did officials decide to use such an anachronistic storage device?

Because it’s still one of the standard methods of delivery. While the use of CDs bemused many in the press, it’s actually not that uncommon for lawmakers to receive sensitive documents on compact discs.

“I left the Hill in 2014, and at that time *any* electronic production was unusual. When it occurred, CD-ROM was what we got,” Justin Rood, a director at the Project on Government Oversight who used to work for Sen. Tom Cotton, wrote in an email to Slate. “When I saw that the report was delivered on CD-ROM, I thought it was fairly typical.” Indeed, CBS reported that staffers on the House Judiciary Committee still have access to working CD drives.

CD-ROMs are a preferred delivery vessel for classified materials in Congress in part because they have a considerable amount of memory and are largely impervious to tampering. As Wired notes, CDs can also typically hold 700 megabytes of data. The 412-page FISA application to surveil Trump campaign adviser Carter Page, for example, took up only 7.8 megabytes of space, so placing the document on a disc would leave you with plenty of room.

Daniel Schuman, the policy director at Demand Progress, told Slate that some congressional offices have access to email networks that are equipped to disseminate classified material and that such documents are occasionally delivered on hard drives as well. “Sometimes information isn’t given to Congress, but you have to go to a secure facility and can read it there,” he added. It was the case for the report that the FBI produced on the sexual assaults allegations against Supreme Court nominee Brett Kavanaugh last year.

As for everyone else: We got a scan of a PDF uploaded to the Justice Department website. It’s not searchable. (Update: We made a searchable version!)

This post was updated once we had converted the scan to a searchable doc.