© 2014 Fuji Technology Press. All rights reserved. The electromagnetic (EM) method first applied to monitor the activity of the Taal Volcano from 2005 consists of repeatedly surveying total magnetic intensity (TMF) and continuously measuring EM fields. The EM observation system was greatly improved through the SATREPS project summarized in this paper. The resistivity structure is investigated in the project using the magnetotelluric (MT) method. The MT method revealed a large hydrothermal reservoir just beneath the center of Volcano Island. The existence of this reservoir explains several things-the special features of the devastating 1911 eruption, i.e., that it was magmatic-hydrothermal, together with the geochemical characteristics of Main Crater Lake, especially the absence of SO2 gas. The reservoir's formation process provides useful suggestions for understanding the volcano's past eruption cycle.