Unless you are talking about bottom as in 10 metres down. That won't protect you in case something goes seriously wrong.
Saying that the thread is about SpaceX and Mars and all I can say is "GO SPACEX!"
Hell, Zuckerburg, Yuri Milner, and Steven Hawking are already planning it:
http://www.theatlantic.com/science/archive/2016/04/yuri-miln...
Maybe. What if it's an artificial plague? What if any other number of catastrophic events cause the collapse of the civilization here?
Having multiple settlements elsewhere is insurance.
And, while Mars is one interesting place, there are other rocks we could easily settle and mine for resources. I don't think Mars should be the first we tackle (I'd go for the Moon first) but I'm perfectly fine with humans spreading all over the solar system.
Ah but once Earth is no longer the single point of failure then the Sun becomes the single point of failure :)
I'm very excited about all things space and I hope SpaceX pulls it off