According to the Energy Information Administration, like this:
The 1991 and 2001 dips were due to economic slowdowns. 2008 is a "flash" estimate and may be revised. The economy may be a contributing factor there, as well. However, the other low bars took place during periods of economic growth.
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There's a Whole in the Bucket
There is a little bit of the Netherlands surrounded by a Belgian enclave.....
that is surrounded by a Dutch enclave in Belgium. The border appears to be fractal. The houses have special door plates to indicate which ones are in which country. Above, the Netherlands is white, Belgium is red.
The Belgian-Dutch border was established in the Maastricht Treaty of 1843, which mostly confirmed boundaries which were a few centuries old (as the separation of Belgium and the Netherlands has its origin in the religious wars of the 16th century). In the area around Baarle, it proved impossible to reach a definitive agreement. Instead, both governments opted to allocate nationality separately to each of the 5.732 parcels of land in the 50 km between border posts 214 and 215.
These parcels ‘coagulated’ into a veritable archipelago of 20-odd Belgian exclaves in and around Baarle. In turn, some of these Belgian exclaves completely surround pieces of Dutch territory. Deliciously complicating this picture is a small enclave of Baarle-Nassau situated entirely within Belgium proper – and there’s even a Belgian parcel within a Dutch parcel within a Belgian enclave, which in turn is surrounded entirely by Dutch territory…