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Stóri Geysir (Great Geyser) – that gave them all a name – hasn't been its former self for more than half a century, when tourists through rocks into it to set it off. But its close neighbor Stokkur is reliable, spouting hot water every few minutes as shown. As you might know, this phenomenon is similar to boiling clean water devoid of air (bubbles): Vapor bubbles can't form without 'help', microscopic vapor bubbles require an enormous internal pressure to expand because of the surface tension, so the water gets superheated. When (with some obscure kind of help) a vapor bubble finally forms, it expands almost like an explosion.