Unlike Hiroshima, Nagasaki lies in a series of narrow valleys bordered by mountains in the east and west. The bomb exploded about 500 m. (1,625 ft.) above the ground and directly beneath it (the hypocentre) was a suburb of schools, factories, and private houses. The radius of destruction for reinforced concrete buildings was 750 m. (2,437 ft.), greater than at Hiroshima where the blast caused by the bomb was more vertical. But because of the topography, and despite the Nagasaki bomb being more powerful, only about sq. km. ( sq. mi.) of Nagasaki was reduced to ashes compared with 13 sq. km. (5 sq. mi.) of Hiroshima. Of the 51,000 buildings in the city % were completely destroyed or burt, with % escaping any damage.