The Sierra de Cartagena-La Union, known as the "Sierra Minera", is a mountainous formation that stretches in an east-west direction along 26 kilometers of the Mediterranean coast, from the city of Cartagena to Cabo de Palos, in the province of Murcia.
There is evidence of intense vulcanization activity that started over seven million years ago and that gave way just a million years ago in the field of Cartagena, at the same time that powerful hydrothermal processes took place in which water emerged on the charged surface of dissolved minerals. These rushed into faults and empty spaces in the mountains, accumulating, in this way, in bulging seams of metallic minerals.
The use of these concentrations of minerals—mainly silver and lead—has been intense since ancient times. This is the reason why the Carthaginians focused on southern Spain and, later, the Romans. However, the exhausting of the most accessible deposits and the limited development of Roman technology led to the suspension of mining activity.
It can be assumed that, originally, the Sierra de Cartagena-La Union was covered with a thick mantle of shrub and evergreen species, typical of siliceous soils.
Basically, there would be hawthorn, wild olive, esparto grass, and mastic. Pines, kermes oaks, and Tetraclinis articulata
would also grow. In the areas of higher humidity, there would be strawberry trees and holm oaks.
These original ecosystems were maintained for centuries in more or less unchanged form while mining was practiced by underground exploitation.
However, with the progressive increase in the population in the vicinity of the Sierra de Cartagena-La Union, mining activity was attracted, particularly from the 18th century onwards. The pressure on the vegetation cover increased with grazing, woodcutting, and breakage, which degraded it and replaced it, already in the 19th century, with an ecologically weak thorn.
At the same time, with the emergence of new technologies starting in 1952, mining was resumed in a highly profitable way, becoming an extensive activity in the form of open-pit mining.
This caused serious damage to the environment due to the great earth movements, with the consequent loss of soil and of numerous species and habitats, creating extensions of sterile land.
On the other hand, severe environmental damage occurred with the dumping of large amounts of mining waste—containing concentrations of heavy metals and toxic products—into Portman Bay. In 1990, mining activity in the mountains stopped due to its low profitability and to pressure from environmental groups.
The traces of human activity and of this intense historical process of exploitation are imprinted on the landscape of the Sierra de Cartagena-La Union. There are also archaeological, cultural, and industrial reminders of this tumultuous mining past. For these reasons, the Sierra de Cartagena-La Union was declared, on April 30, 2009, a Property of Cultural Interest by Decree of the Governing Council of the Autonomous Community of the Region of Murcia, with the category of historical site.
Despite the negative environmental impact produced by mining, the Sierra de Cartagena-La Union still has important natural resources.
In the eastern part of the mountains, from Portman to Cala Reona, the area of Calblanque, Monte de las Cenizas, and Peña del Aguila has been protected, under the categories of Regional Park and Site of Community Importance (SCI).
In addition, the International Union for Conservation of Nature (IUCN) has identified it as a Protected Area with Sustainable Use of Natural Resources corresponding to category VI of the IUCN system.
Also, west of Portman is the Sierra de La Fausilla, declared a Special Protection Area for Birds.
Finally, to protect the exceptional plant diversity of the mountains, the declaration of seven botanical micro-reserves has been proposed: five in the municipality of Cartagena and two in the municipality of La Union.