As the Channel Tunnel, one of the modern world's great engineering feats, inches toward completion, its builders are forced to address a basic question preying on the minds of prospective travelers. Is it safe?

"Few incidents or breakdowns are expected and the vast majority of these are likely to cause delay rather than danger," according to a brochure from Eurotunnel, the Anglo-French consortium building the underwater tunnel.Much of the technology used to dig the tunnel is tried and tested. But air, rail and ferry disasters in recent years have reminded the consortium - and its potential customers - that operating and safety technology must be state-of-the-art.

Fire, flooding, derailment or terrorist attack in the 30-mile link could be a nightmare. So Eurotunnel is taking every precaution for the safety of an estimated 28 million people expected to use it annually after it opens in 1993.

A milestone was reached in its construction Tuesday when tunnelers working from each side of the English Channel pushed a metal probe through a bore hole just two inches wide, tenuously linking Britain with the European continent.