What is a .arc file?

A .arc file is an Internet Archive (version 1.0). This is an archive — a container holding other files, usually compressed. Confusingly, this extension has been used by 3 different formats over the years — they're all listed below.

This extension has been used by more than one format

3 different formats have claimed .arc over the years, so two files with the same extension aren't necessarily the same kind of thing.

How to open a .arc file

7-Zip (Windows) or Keka / The Unarchiver (Mac) extract most archive formats, including old ones.
On Linux, try `7z x`, `unar` or Ark.

The registry records this extension against Internet Archive, Internet Archive, FreeArc Archive Format, so software from that family is the most likely match.

How to convert a .arc file

Extract the contents, then re-compress as .zip if you need to share them widely.

MIME type

When a .arc file travels over the web, servers label it with this MIME type:

application/x-internet-archive

What's a MIME type?

The technical record

For archivists, developers and the merely curious: these are the PRONOM registry entries for .arc.