If you take a bunch of random particles and put them together, why should a pole form on each side of this collection?
Some particles already have a magnetic field. Many particles are polar, such that they will orient themselves in a magnetic field. If you jumble them all together, they will self align, and eventually one strong field will be externally detectable even though their individual fields were small and unorganized at the start.
Perform this experiment: Drop a bunch of magnetic powder and dirt into a bag. Shake vigorously. What is the resulting clump's magnetic signature?
Is it in practice possible to create a device capable of canceling the earth's magnetic field in a region the size of the north sea?
No. What you want is a Helmholtz coil, adjusted electronically to react to the earth's changing field.
However, the area of the field required, even though it would be relatively low magnetic force, would require entirely too much energy to be practical. Further, an ideal Helmholtz coil, where the field is uniformly 0 everywhere inside the coils, requires essentially a cubic structure. The North Sea is 970 KM long, and thus the coils would need to be 970KM in diameter, vertically oriented, buried a significant portion of that depth into the ground on either side of the north sea.
Further, it would really mess up the compasses of people traveling anywhere near the coils, not to mention other animals that appear to depend on magnetic fields, such as some migrating birds.