I just watch a documentary about the nature of reality (horizon) there seems to be allot of excitement about the Higgs boson that apparently holds the key to mass. however I think I read somewhere that as a you approach the speed of light you gain mass and I'm sure iv read about mass being gained because of energy. does this no suggest that there is no such thing as mass and in fact mass does not exist and is just a measure of energy?

Hypothetically, if you had a gas made of really heavy atoms/molecules (although it's unlikely that they would be stable at the kind of Uranium-heavy I'm thinking of, let's say that hypothetically they are) in a liquid made of really really light molecules or atoms (e.g hydrogen), and say that temperature isn't going to change the given states of the elements (very very hypothetically), and so they can exist in the states they need to together, could the bubbles go down? Or would the relative densities make the masses of the atoms and molecules irrelevant? 

According to Einstein's famous equation E = MC2, or Energy = Mass times the speed of light times the speed of light. This tells us that anything that has mass, such as matter, is comprised entirely of energy. Any physical object with mass is therefore simply a lump of energy existing in some stable form.

As I understand it, all forms of energy, including chemical energy, kinetic energy and potential energy are covered by Einstein's equation, meaning that fundamentally all forms of energy are the same basic 'stuff' (for want of a better word). My question is, therefore, what exactly is energy? I want to get to a fundamental understanding of energy, because it seems that such an understanding would help to explain much of reality.

I'm not asking for explanations about work done etc, I'm asking at the fundamental level what is energy?

Excluding any blackholes or other exotic phenomena, how small would the earth become if you removed all the empty space from its atoms (approx of course). I did email these questions to newscientist years ago, but never did get any feedback.