The Queens Midtown Tunnel is a shield-driven subaqueous vehicular tunnel, 31 feet in diameter, driven with the aid of compressed air. It crosses beneath the East River in the city of New York and joins the boroughs of Queens and Manhattan. The article describes (1) the preliminary explorations for the project, (2) the kinds of soil and rock penetrated by the tunnel, (3) the problems encountered in driving the tunnel, and (4) the relations of the geologic conditions to the design and construction. The purpose of the article is to familiarize engineering geologists, unacquainted with subaqueous tunnel v/ork, with the practical aspects of such work, and with the geologic features which require attention during both exploratory investigations and construction.