Grotto Geyser, the namesake and most important geyser of the Grotto Group, lies at the fork of the old road with the boardwalk to Giant Geyser. No other geyser cone is even remotely comparable to Grotto's. It is supposed that the weird shape is caused by geyserite coated tree trunks.
Grotto erupts at intervals of 6 to 8 hours. Besides the normal eruptions, lasting 1 to 3 hours and reaching up to 10 m (33 feet), also long-lasting eruptions of more than a whole day are known, called marathon eruptions.
Grotto's neighbor to the north is Rocket Geyser. Usually Rocket erupts simultaneously with Grotto.
Except for Variable Spring east of the boardwalk to Giant Geyser, all other members of the Grotto Group are located north of Grotto. Variable Spring shows superheated boiling during Grotto's marathon eruptions, otherwise it is a quiet spring.
A cluster of features, strongly linked to Grotto's activity, can be observed on a clearance north of Rocket Geyser. The gaping hole in center is the crater of Grotto's Indicator Spring, also called Grotto Pressure Pool, showing intermittent rising and falling of the water level during hours before Grotto erupts. Left beyond Indicator Spring the eruption column of Grotto Fountain Geyser is visible. It often erupts short before Grotto starts to play and the slender water jets may reach impressive heights.
Simultaneously with Grotto Fountain Geyser the smaller South Grotto Fountain Geyser plays up to 3 m (10 feet) high.
Spa Geyser appears next to the fork, where a small trail branches to Riverside Geyser. As an intermittent spring it fills and overflows during Grotto's play. Eruptions of Spa occure only on rare occasions.
On the way to Riverside Geyser you pass Marathon Pool right of the trail. The name refers to Grotto's marathon eruptions, when the water level in Marathon Pool may drop slightly.
Riverside Geyser undoubtedly belongs to the most pictorial geysers of Yellowstone. It is erupting very regularly every 6.5 hours, and variations typically range in the limit of 30 minutes. The main vent lies on the bottom of the elevated basin, but also the hole in the raised cone is an active vent.
The next photos show the two described vents of Riverside Geyser in action.
Across the river and the trail from Riverside Geyser there are some small intermittent springs embedded in the vegetation patch in between the trails. UGGNN005 stands out due to its nodular sinter rim.
Not far away to the north the circular bowl of UGGNN007 appears.