The Ancient Bath is the eighth puzzle in Virtual Villagers 3: The Secret City.
The task in this puzzle is to have your Master Builder write on the chalkboard the plans that are needed to fix the bath.
You cannot attempt this task until you have a Master Builder and your Alchemy Lab has been fixed.
Drag and drop your Master Builder onto the white board that is in the Alchemy Lab and they will begin to draw the plans that are needed to fix the bath.
Once the plans have been written, drop any of your villagers onto the bath so they can begin repairing it.
You'll see the villagers that you drop to repair the bath start going down the steps, it doesn't mean that they are leaving. It means that the repairs they need to do are on the outside of the bath not on top.
This process can take a very long time to complete, make sure that your other workers are taking care of things that they need to be doing.
Once it's repaired your villagers can take a swim in the pool at any time.