The geomagnetic total intensity data recorded by a geomagnetic network in Taiwan comprised of 11 permanent proton magnetometers was used to compute diurnal variation ranges to examine if any discrepancies were present. Diurnal variations with similar ranges were obtained from all stations except for the HC station. The ranges at the HC station are often two times greater than those recorded at the other stations. This is an interesting phenomenon that requires considerable attention. Eight temporary stations were set up between the Taitung (TT) and Hengchun (HC) stations, distributed evenly over the Hengchum peninsula during 13 September 2012 to 25 January 2013 to clarify the phenomenon. The local stations in Taiwan are compared with 2 international stations to explain the diurnal ranges resulting from the interaction between solar activities and the Earth’s main geomagnetic field and/or local effects. Analytical results show that the unusually large diurnal ranges were observed mainly on the Hengchun peninsula. The surrounding the sea water produces the magnetic coast effect.