Tunnel nine

The Fourth Doctor stands in tunnel nine as it is fumigated. (TV: Underworld)

Tunnel nine was an area on the planet P7E. Following Idas' escape into the tunnel, the guards had it fumigated. The Fourth Doctor ventured out into the tunnel and the gas began to choke him. The guards were forced to drain the gas away when it began to seep into their base. It was extracted from the tunnel, saving the Doctor's life. (TV: Underworld)