Thrusting up 151 metres, the headquarters of the Nationale-Nederlanden insurance group was from 1991 till 2010 (the Maas Tower the country's tallest office building. Designed by the Friesian architect Abe Bonnema, it was the winner out of five entries to an invited competition held in 1986. The brief was to design two office buildings of 36,000 and 24,000 square metres to be used independently by Nationale-Nederlanden and its then recent acquisition the RVS life insurance company respectively. The two share a basement that includes the entrance, a company restaurant, a training and conference centre and a sports hall; the brief also called for a parking facility for 1200 cars. This basement takes up the entire plot. The two office towers of 151 and 93 metres stand on columns either side of the metro running beneath the site on a table-like structure. They have a clear span of 14.4 metres. The facades consist of structural members in precast concrete with a finish of solar reflective glazing. Cores each containing a lift and stairs and external buttresses provide stability. These are clad in black neoparies, a product of crystallized glass as hard as granite.