Mount Yasur is a volcano on Tanna Island, Vanuatu, 361 m (1,184 ft) high above sea level, on the coast near Sulphur Bay, northeast of the taller Mount Tukosmera, which was active in the Pleistocene. It has a largely unvegetated pyroclastic cone with a nearly circular summit crater 400 m in diameter. It is a stratovolcano, caused by the eastward-moving Indo-Australian Plate being subducted under the westward-moving Pacific Plate. It has been erupting nearly continuously for several hundred years, although it can usually be approached safely. Its eruptions, which often occur several times an hour, are classified as Strombolian or Vulcanian. A large lava plain creeps across the valley at the base.

The glow of the volcano was apparently what attracted Captain James Cook on the first European journey to the island in 1774. Today, the mountain is a sacred area for the John Frum cargo cult. Members of the cult revere John Frum, a deified messenger who foretold the bringing of wealth to the island by American forces, and believe he resides in Mount Yasur with his countrymen. The village of Sulphur Bay, the center of the movement, claims the volcano as part of their territory.

In respect of it 'erupting for more than 800 years' and being 'the lighthouse of the Pacific!', the International Union of Geological Sciences (IUGS) included 'The active Yasur–Yenkahe volcanic complex' in its assemblage of 100 'geological heritage sites' around the world in a listing published in October 2022. The organisation defines an IUGS Geological Heritage Site as 'a key place with geological elements and/or processes of international scientific relevance, used as a reference, and/or with a substantial contribution to the development of geological sciences through history.'

Mount Yasur is an easily accessible active volcano, and is a major Vanuatu tourist attraction. The Vanuatu Government monitors the level of volcanic activity in the interests of the public, both tourists and locals alike. This monitoring is carried out by the Vanuatu Geo-Hazards Observatory.

Unbelievably Vanuatu Post has established a post box near the fiery rim of Mt.Yasur's crater, where mailing a letter is a hot experience! Imagine molten lava exploding in the air metres from where you are writing and sending your postcard! This is what Volcano Post is all about.

The postal outlet on Mt.Yasur is recognised as the only post box in the world on a live volcano and has the support of Vanuatu's National Tourism Office. The post box is officially designated by Vanuatu Post as a postal outlet and mail is collected regularly by appointed postal agents. All mail receives a unique Volcano Post cancellation, which will become sought-after items amongst the world's stamp collectors. Like the Underwater Post Office, Volcano Post has dedicated stamps and postcards that enable the visitor or collector to keep these unusual and hard-to-find items. Imagine showing your friends a postcard from the Volcano Post Box, which was sent to you just a lava rock's throw from molten lava with temperatures over 1000°C!