A group of scientists and nature lovers exploring tunnels in the Canary Islands became trapped underground and at least six of them died after apparently inhaling toxic gases, officials said Sunday.

The group of 30 — among them scientists from a renowned astrophysics observatory — descended into the manmade tunnels Saturday. Several people appeared to have been struck unconscious by gases, said Jose Miguel Ruano, Tenerife regional government minister. Six people were pulled from the tunnel dead, and six other were taken to hospital by helicopter, Ruano said.

Rescue efforts were made difficult because of the gases and cramped conditions underground, Ruano said.

The island's emergency services director, Jose Andres Garcia, said the group was about a mile underground in an area where gases may have seeped into the tunnels, robbing them of oxygen.

One person managed to make it out of the complex known as Los Silos and alerted emergency services, Ruano said.

Among the group were scientists from the Canary Island Astrophysics Institute and members of the Tenerife Friends of Nature Association.

Tenerife, a volcanic island, is located in the Atlantic southwest of mainland Spain. The tunnels were created to prospect for water.