Could you use earth's magnetic field to build a hover skateboard? In "Back to the Future" they use levitating anti-gravo skateboards without wheels which will be difficult to build, because there is no theory on how anti-gravity could be created.
A possible solution in the far future could be to use Earth's magnetic field to create a kind of induction field that keeps the skateboard above the ground.
What would a theory look like to create such a field?
 A: When superconductivity was first discovered this was one concept that was quickly explored.  Unfortunately the weak field of earth requires a massive circulating current to generate a counter magnetic field with enough force to lift something off the ground.
There are two limiting factors: 


*

*The need for more current means more weight which means more current... 
In fact the first pass analysis shows you need power on the order of
20kW, which is about the same amount of power generated by a locomotive engine.

*The Earth's magnetic field is not perpendicular to the surface, in
fact near the equator there's almost zero vertical component to work
against.  This angle reduces the amount of available field strength
to lift against gravity.


Perhaps one day if we can develop massive yet micro-sized power sources something like this might be possible, but away from the equator.
Edit:  So how this concept works is first look at the vector of how the magnetic field on the earth's surface (away from the equator).  There is a vertical component that is parallel to the force due to gravity.

A loop of superconducting current (assuming there is a way to get it started) would create a field so that if you set it perpendicular to the earths surface the magnetic moment would oppose the vertical component of the earth's field.
The torus fields looks like this.

I unfortunately need to run, but the calculations are similar to superconductor levitating in earth's magnetic field? but for a loop and not just to levitate itself, but additional mass.
A: I did not quite understand the idea that user6972 discusses and how (s)he could use the vertical component of the Earth's magnetic field, however, one could use the horizontal component of the field. For example, if you have a (closed) superconducting cable with a large current around the equator, it can have some lift in the Earth's magnetic field, and it does not need power (other than to maintain sufficiently low temperature). It's more difficult to provide lift for an arbitrary trajectory (so this is probably not for scateboards:-) ), but it can be done in principle, say, using large closed superconducting cables with current and the difference between Earth's magnetic field at different altitudes. Of course, these are hypothetical possibilities, as the Earth's magnetic field is relatively weak, as user6972 pointed out. 
