The Taiwan Strait tunnel, as a great project, has been studied for a long time and met many challenges from the beginning of the feasibility study that has attracted attention among scientists and engineers on both sides of the Taiwan Strait and around the world as well. The key question is whether this undersea tunnel can be constructed with present technologies under such complex geologic and seismologic conditions. Results of current researches indicate that the sea floor of the Strait is covered with a horizontal layer composed of both the Pleistocene and the Holocene sandstone and shale with a thickness of about 200～300 m, without any large fault belt or fold.This can serve as a good bearing and impermeable layer for constructing such an undersea tunnel. The Penghu Islands and Penghu terrace, which are composed of hard basaltic rock over a wide area of 70 km (NS)×40 km (EW), might be connected each othar by a bridge instead of an undersea tunnel, and serve as a construction site. Although there are several geologic units separated by faults under the upper horizontal layer, these faults do not cut through the upper layer. Only the NE offshore fault along the Fujian coast and that along the west coast of Taiwan should be noticed, between which a suspension tunnel instead of an undersea tunnel can be constructed. Therefore it is judged that the tunnel beginning from Amoy, through the Jinmen Island and the Penghu Islands to the Peigang harbour is worth recommendation. The Taiwan Strait tunnel, as a great project, has been studied for a long time and met many challenge from the beginning of the feasibility study that has visited scientists and engineers on both sides of the Taiwan Strait and around the world as well. The key question is whether this undersea tunnel can be constructed with present technologies under such complex geologic and seismologic conditions. Results of current researches indicate that the sea floor of the Strait is covered with a horizontal layer composed of both the Pleistocene and the Holocene sandstone and shale with a thickness of about 200 ~ 300 m, without any large fault belt or fold. This can serve as a good bearing and impermeable layer for constructing such an undersea tunnel. The Penghu Islands and Penghu terrace, which are composed of hard basaltic rock over a wide area of ​​70 km (NS) × 40 km (EW), might be connected each othar by a bridge instead of an undersea tunnel, and serve as a constructio n site. Although there are several geologic units separated by faults under the upper horizontal layer, these faults do not cut through the upper layer. only the NE offshore fault along the Fujian coast and that along the west coast of Taiwan should be noticed, between It a judged that the tunnel beginning from Amoy, through the Jinmen Island and the Penghu Islands to the Peigang harbor is worth recommendation.