Nathaniel P. Langford described the Grotto in his 1871 Scribner's account: "The Grotto" was so named from its singular crater of vitrified sinter, full of large, sinuous apertures.Through one of these, on our first visit, one of our company crawled to the discharging orifice; and when, a few hours afterwards, he saw a volume of boiling water, four feet in diameter, shooting through, it to the height of sixty feet, and a scalding stream of two hundred inches flowing from the aperture he had entered a short time before, he concluded he had narrowly escaped being summarily cooked.Approximately three-quarters of Grotto's eruptions are short mode.Additionally, the long, "marathon" eruptions of Grotto are commonly followed, a few hours after they end, by a particularly intense "recovery hot period" in the Giant complex that may instigate a Giant eruption.The odd shape comes from sinter accumulating over dead tree stumps as the geyser erupted.