At some occasions in the past, tunnel solution was only an alternative for bridge solution in planning corridors crossing rivers and seas. In the paper, the advantages, features and problems of building underwater tunnels are discussed, which provides a complete and objective view on the function of underwater tunnels in construction of corridors crossing rivers and seas. Typical underwater tunnels built, under construction and under study are listed, and river/sea bed terrain, engineering geology, hydrogeology and other site conditions, where bridges, tunnels or “bridges + tunnels” are suitable to be built, are proposed. Analysis is made on the bridge solution and the tunnel solution in terms of advantages, disadvantages, applicable conditions and its limitations. Some typical underwater highspeed railway and highway tunnels are listed, the favorable conditions to build underwater tunnels are summarized and the problems in the construction and operation stages of underwater tunnels are analyzed. Conclusions drawn are as follows: In planning corridors crossing rivers and seas, comparisons shall be made among different alternatives, considering the natural conditions, ecological conditions, geological conditions, hydrological conditions, river conditions, harbor conditions, waterway conditions and shipping conditions of the site, so as to find out the optimum solution. Underwater tunnels have their own advantages and disadvantages. Due to the unique technological and economical advantages, in fact, underwater tunnels have already been adopted in more and more corridors crossing rivers and seas.