1275 ANNALS OF GEOPHYSICS, VOL. 49, N. 6, December 2006 Key words Colli Albani volcano – Albano Lake – multibeam bathymetry 1. Introduction The Colli Albani volcanic complex occupies a wide area about 25 km SE of Rome. Its gener- al structure is a caldera with a central cone. This complex displays two nested calderas and sever- al more or less eccentric post-caldera vents, most of which have been produced by explosive activ- ity. The highest point is Monte Cavo at 949 m, which consists in a scoria cone located in an ec- centric position on the SW rim of the younger Faete caldera. The two crater lakes of Albano and Nemi fill the most recent craters of the vol- cano (fig. 1) (Trigila, 1995). The former geological studies performed by Mercalli (1883), Fornaseri et al. (1963) and De Rita et al. (1988, 1992) dated all deposits of the Colli Albani at an age older than Holocene (>10000 years). Until recent times, they have been considered an extinct volcano, although some historical documents reported some eruptive ac- tivity around 114 B.C. (Funiciello et al., 2002) and 7000 years BP (Andretta and Voltaggio, 1988). In recent times, evidence of an ongoing volcanic unrest based on instrumental seismolog- ical and geodetic data have been detected (Ama- to and Chiarabba, 1995; Chiarabba et al., 1997; Anzidei et al., 1998), and new researches indicate that an eruptive activity likely occurred during the Holocene (Funiciello et al., 2003; Porreca et al., 2003), as also previously suggested by Andretta and Voltaggio (1998) and Villa et al. (1999). Measurements of ground deformation avail- able from high precision levelling lines estab- lished by IGM in 1951, as well as other bench- marks measured at the end of the last century, in- dicated a uplift at 30 cm in 43 years at a rate at ∼ 0.7 mm/yr (Amato and Chiarabba, 1995). The broad deformation zone evidenced through Din- SAR observations by Salvi et al. (2004), mainly across the two lakes of Albano and Nemi, was in- terpreted as related to a superficial source (3-6 The dark side of the Albano crater lake Marco Anzidei ( 1 ), Alessandra Esposito ( 1 ) and Francesco De Giosa ( 2 ) ( 1 ) Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy ( 2 ) Coastal Consulting and Exploration, Bari, Italy Abstract The Albano Lake is the deepest volcanic lake among the volcanoes located in the Italian Peninsula. It belongs to the Colli Albani volcanic complex whose last largest eruptions are dated back to about ∼30 Kyr, although mi- nor events likely occurred during historical times at 7000 years BP or earlier. After the end of the volcanic ac- tivity the Crater of Albano became a lake whose level changes have been known since historical times. In No- vember 2005, the first very high resolution bathymetric survey of the Albano Lake was performed by means of a multibeam echo sounder, integrated with the GPS/RTK positioning technique. Special effort was devoted to produce a high resolution morphobathymetric map, which aims to provide a Digital Terrain Model of the lake floor for wide applications. The surveys did not revealed significant gas exhalative centres, which should indi- cate a current active gas release from the lake floor. Here we show the technical details of the bathymetric sur- veys, the very high resolution bathymetric map and the main morphological features of the Albano Lake bottom. Mailing address: Dr. Marco Anzidei, Istituto Naziona- le di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143 Roma, Italy; e-mail: anzidei@ingv.it