In the 11th chapter of Life on Kepler Bb after the whole sequence of earthquakes crumbling all the tunnels and making the homes and buildings dusty a geologist named Leon finds out that the fault line is very deep and very hot. Leon tells all people with the power of flight to line up in groups of 4 and everybody else to get out of the homes, buildings, and tunnels fast whether that requires digging or not. The groups of 4 people each carry a certain amount of people far away from the fault line by flying. Some of them have skin flaps and others have wings. They all curl their toes inward using the plantar fascia and the flexor digitorum brevis to hold the thing they are carrying the people on with their feet while they fly sort of like bird claws. It happens to be about 100 miles away towards a mountain range. They now see the main river called the Black Sword River. There is a lake a few miles away on one of the branches of the river called Big Walnut Lake. It is a very deep lake just like how the fault line is very deep. But it isn't as deep as the fault line is. I could imagine the flying humanoids getting very tired almost to the point of falling after 100 miles of flight. If the civilization is mostly underground, almost inevitably are some tunnels going to go through the lake or the river. This is just as big of a problem as the lava is because water is going to flow into the homes and buildings eventually draining the river and lake completely. To overcome this they need to do this: A: Have barriers at the ends of the tunnels at the lake and river that water can't get past. B: Dig deeper at the risk of forming lava tubes and eventually underwater volcanoes. C: Build tunnels above the water that lead to underground tunnels or D: Go completely above ground Advantages: A: Slower rate of draining the lake and river B: No draining of the lake and river C: No risk of lava tubes and underwater volcanoes No draining of the lake and river D: Almost no risk Disadvantages: A: Barriers won't work in the long term even if water wasn't drained out of the lake and river because of rotting or oxidation B: Lava flowing into the buildings and homes Tunnels being too hot for young ones even without the lava. C: Takes some getting used to D: Almost impossible to accommodate for As you can see each choice has its advantages and disadvantages. B is the worst of them all. The humanoids evacuated because lava tubes would form and they don't want the risk of lava a second time if possible. I would say C would be the best both in terms of lifestyle(These humanoids climb up and down tunnels all the time to get from place to place and store a mental map of the whole civilization) and no risk of the lake and river draining out or lava(Water is important to them both for food and for drinking)(Lava is their enemy). But would C actually be the best choice or would it be D that is best or something completely different?