Ancient Ruins in Civilization V are remnants of extinct civilizations, mementos of past greatness and order amid the chaos of the wilderness. Budding "contemporary" civilizations may learn a lot from these places, which will help them in their early stages. Not to be confused with City Ruins.
Game Info
When landed on by any unit, Ancient Ruins give a reward to the owner of the unit. This is a one-time bonus, after which the ruin disappears. These rewards include:
- A crudely drawn map of the surroundings of the ruins (Reveals some of the nearby tiles; note that the center of the revealed space is not the ruins tile, but somewhere 3-4 tiles away in a random direction)
- Evidence of Barbarian activity (Reveals location of one or more nearby Barbarian encampments)
- Secrets of the ancients (Gives a free Ancient Era technology; note that all prerequisites for the technologies have to be met, so a civilization can't get one of the more advanced techs - that is, the ones on the right-hand side of the Ancient Era tree - until its prerequisites have been met)
- Survivors, which join the civilization (Adds 1
Population Population to any one of its cities) - Advanced weaponry (Gives a free upgrade to the unit used to activate the ruins)
- Gold treasure (Gives approximately 50-100
Gold Gold) - Cultural artifacts (Gives approximately 10-20 Culture Culture)
- Religious artifacts (Gives 30 Faith Faith)
- A prophecy (Gives 80 Faith Faith; note that the civilization must have founded a Pantheon before this reward becomes possible)
And only on the easier difficulties:
If the game pace is set on Marathon, the amount of 
Culture Culture and 
Faith Faith found is greatly increased, but not the 
Gold Gold.
The choice of reward is always random, but it seems that sometimes some ruins are bound to a particular benefit - for example, unit upgrade - and will always bestow it upon activation. Also, some benefits can only be activated after receiving another Ancient Ruin benefit - for example, the Prophecy benefit may appear only after getting the Religious artifacts from another ruin.
Finally, it seems that all Ancient Ruins have the full range of possible rewards; however, you cannot get a reward that you have already obtained from the last two ruins you found. This is best seen when using the Pathfinder's ability, which gives direct access to each particular ruin's list of rewards.
Thanks to the new archaeological system in Brave New World, Ancient Ruins will later spawn Antiquity Sites in the locations where ruins were encountered and activated, so Archaeologists may return there later to dig and uncover more valuable secrets.
Strategy
An Ancient Ruin in-game
The discovery of Ancient Ruins is vital for the early development of a civilization. The bonuses might seem small, but for the very beginning of the game they are very significant. A single citizen may considerably speed up the development of a city. The Culture Culture bonus may instantly give a Social Policy (or speed up acquiring the next one). The Faith Faith bonus may allow for starting a Pantheon even without constructing any Faith Faith-generating buildings. Also, upgrading Scouts gives Archers, even at a time the civilization lacks the technology to build any of these yet, while upgrading Warriors yields the more powerful Spearmen, allowing them to fight Barbarians more effectively. And of course, a free technology is always welcome.
In Brave New World, the Shoshone civilization's unique unit, the Pathfinder, may choose which of the rewards to activate upon entering the Ancient Ruins. Needless to say, this allows choosing the path of development, although as with normal ruins you cannot choose one that you got in your last two ruins.
All in all, the discovery and activation of Ancient Ruins will give a civilization a crucial advantage in the early game. It should become an integral part of early game exploration, and it's highly recommended, regardless of which civilization playing as, to consider building a Scout first upon settling the capital, and use it along with the initial Warrior unit to explore as far as possible.
Historical Info
Ancient Ruins are the remains of earlier long-dead civilizations. They are filled with valuable secrets - gold, maps, lost technologies and sometimes even survivors - awaiting discovery by the intrepid explorer. Rich rewards await those who first enter an Ancient Ruin.