Lewis & Short

Amp-sanctus (better than Am-), i, m., a lake in Italy, highly dangerous from its pestiferous exhalations (hence, in the poets, the entrance to the infernal regions), now Le Mofete or Lago d’ Ansante, Verg. A. 7, 565; Cic. Div. 1, 36; Plin. 2, 93, 95, § 208.