See the context of this sign.

The "Fire Hole"
Geyser Basin Discovery

When the Washburn expedition rode into the Upper Geyser Basin
on September 18, 1870, the explorers were unprepared for the
strange sights that awaited them. Until then, there were only a
few wild rumors of a "fire hole" seen by Indians and mountain
men. The expedition camped here for twenty-two hours and
witnessed a dozen geysers eruption, including one that General
Washburn named Old Faithful.

Within a few years the area became a tourist attraction, with
buildings, road, and crowds. To recreate the experience of the
Washburn expedition, return in winter or at dawn or dusk to
experience the geyser basin's primeval quality.

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