Complex sentence: Perlan is situated on the hill Öskjuhlíð where there had been hot water storage tanks for decades but in 1991 the tanks were updated and a hemispherical structure placed on top.
Split into simpler sentences: Perlan is situated on the hill Öskjuhlíð where there had been hot water storage tanks for decades. In 1991 the tanks were updated and a hemispherical structure placed on top.