Especially in the metal (iron) underground pipes, along them considerable portion of electricity, of course, carried out some time, remained some "residual magnetism" (More info will be here).
Hasa katika chuma mabomba (chuma) chini ya ardhi, pamoja Them makubwa sehemu ya umeme, bila shaka, uliofanywa wakati, imebakia baadhi ya "mabaki ya magnetism" (Zaidi info itakuwa hapa).