The 4077th actually consisted of two separate sets. An indoor soundstage at 20th Century Fox and an outdoor set in the mountains near Malibu, California (Calabasas, Los Angeles County, California). The Malibu set was used for most exterior and tent scenes for the famous TV show.
On October 9, 1982, just as the series was wrapping production, a major brush fire destroyed most of the outdoor set. The fire was written into the final episode as a forest fire in South Korea caused by enemy incendiary bombs.
The Malibu set is nestled in Malibu Creek State Park. Formerly named the Century Ranch and owned by 20th Century Fox Studios until the 1980s, the site is returning to a natural state, and is marked by a rusted, burned Jeep and an ambulance used in the show.
The hike from where you park your car to the site is a mile. The surrounding canyon walls make for a peaceful and beautiful stroll.
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As you see in the video, the M*A*S*H fan community has helped the site stay preserved. In addition to the signpost and vehicles, there are white ropes set up that outline where each set piece was located, such as The Swamp and the hospital.