Las Ruinas de Xochiquetzal were ancient ruins in Central Heredia, Costa Rica.
The ruins were named for the Aztec goddess of fertility and beauty Xochiquetzal, who was said to have always been accompanied by birds and butterflies, as well as Quetzalcoatl. The ruins were not restored at all, so many of them were covered in plants.
- Main article: Peace Walker Incident
The Militaires Sans Frontières passed through Ruinas de Xochiquetzal during the 1974 Peace Walker Incident. The ruins led to the hidden AI laboratory of Peace Sentinel, built within a Mayan temple. They had to backtrack through the ruins, however, when it was discovered that Huey Emmerich's ID card had been locked out of the lab's gate system, thus forcing them to locate a guard in possession of a different card.
Behind the scenes
Dummied content for the game revealed that the level would have been subject to several Extra Ops missions that weren't included in the final version, more specifically taking a photograph of a ghost and a target shooting contest.
Head for the Lab
- "Miller: Snake, infiltrate Dr. Strangelove's research lab and either deactivate or destroy the AI. The lab's built into some ancient ruins, so find those first!"
- ―Mission select briefing for "Head to the AI Lab"
Locate the ID Card
- "Miller: Snake, to gain access to the lab you'll need an ID card. You can get one from a guard in an orange jacket stationed in an area where you can hear quetzals singing. To get the ID card from the soldier, do a body check. You've got to get inside that lab before the AI gets shipped out. Get a move on!"
- ―Mission select briefing for "Locate the ID Card."
 Perfect Stealth
- "Miller: Time for your mission briefing. This is going to be a tough one. Be ready for a protracted fight. I know you can do it. Get out there and kick some ass!"
- ―Mission selection briefing for " Perfect Stealth"