Generally, "ancient ruins" is not capitalized. It's a common noun phrase describing a general concept.
However, you might capitalize it if:
* It's part of a proper noun: For example, "The Ancient Ruins of Petra" refers to a specific location.
* It's used as a title: "Ancient Ruins: A History" would capitalize the phrase for stylistic reasons.
Example:
* Not capitalized: "We visited some ancient ruins in Greece."
* Capitalized: "The Ancient Ruins of Angkor Wat are a must-see."
Let me know if you have a specific sentence in mind, and I can give you more tailored advice!