Did you hear the big news? Great Britain is no longer an island!

No, there hasn't been a cataclysmic shift in the earth's crust. But there has been a potentially major shift in some of the world's economic and political topography.It happened Tuesday night when the two ends of an underwater tunnel linking Britain with France were joined beneath the middle of the English Channel.

As of now, the breakthrough is largely symbolic because the link consists only of a bore hole about two inches across connecting two larger excavations. Much work remains to be done. By January, however, the channel tunnel, or chunnel as it is usually called, should be wide, and firm enough for British Prime Minister Margaret Thatcher and French President Francois Mitterrand to meet and shake hands in it. By June 1993, trains are scheduled to run both ways through two 32-mile-long tubes. A third tube is to be used for servicing the tunnel.

The basic objective of the tunnel is to enable passengers to travel between London and Paris in about three hours. That time is comparable to flying, if transport to and from airports is included, and is half the time of a car-ferry journey.

More important, by reducing shipment time and possible freight costs, the tunnel can be expected to stimulate trade between Britain and France plus the rest of Western Europe. Moreover, with increased trade and travel should come increased unity and cooperation between Britain and Europe.

Down that path could eventually be found the creation of a United States of Europe, which remains an elusive but alluring dream even though Prime Minister Thatcher scorns it.

Even if that dream is never realized, the chunnel is a prodigious feat of engineering - and, with a price tag of $16.7 billion, a prodigious feat of financing.

The challenge now will be to surpass those technological and economic accomplishments with the more demanding political and social accomplishments that are required if Europeans are to live in harmony as improved transportation and communication bring them physically closer together.