This is the world's most assessible volcano. Your accommodation host will arrange for a vehicle to drive you right up to within about 100 metres of the crater's rim. You walk up a narrow path on the cinder crater wall and then stand on the side of the crater rim considered to be "safe".
You stand there, listening to the hissing noise of steam escaping from vents in the crater. You are looking right down into the molten lava. After a few minutes you feel the edge of the crater begin to tremble under your feet. There is a rushing sound like an express train is heading up from the depths of the earth right towards you followed by a deafening explosion that resonates with every cell in your body.
This will give you an entirely new meaning to the word awesome.
The Yasur Volcano is located on the south eastern part of the island and the excursion to visit the volcano takes at least two hours each way from the airport (depending on the condition of the roads).
Allow for a minimum of three days in Tanna so you have the best chance to see the volcano with good weather conditions.
You really want to visit the volcano in the evening when the exploding lava glows in the sky like fireworks.
From time to time Yasur becomes so violent that visitors can only watch it from the surrounding plains and can't go up on the crater at all. To find out the status of the volcano visit the Vanuatu Volcano Alert site.
To make this easier and more fun - you should plan to stay between three days and a week and consider staying in one of the bungalows on the south eastern side of Tanna, closer to the volcano, hot springs, beach and waterfalls.
You can check out the locations of the village bungalows on the Rocket Guide to Vanuatu. This is an interactive program that gives you a complete understanding of every aspect of Vanuatu tourism (It was designed for educating travel agents). You can download a copy from their site or order the program on DVD - it works on both Windows and Mac.