Wallender Born, Cold water geyser in Wallenborn, Germany.

Wallender Born is a cold-water geyser in Wallenborn, Vulkaneifel. Its artificial shaft sits beneath the surface of a small pond, so water is thrown up turbulently rather than as a fountain. The driving gas is carbon dioxide, with traces of hydrogen sulfide giving a smell of rotten eggs. Eruptions occur about every 35 minutes, first a 2–4 m water column, then several minutes of boiling-looking turbulence.

The Wallender Born was a periodic mofette, a gas-emitting spring, long before any drilling. In 1933 the mofette was bored to a depth of 38 m for mineral water production; the bore became eruptive, throwing out water, drilling mud and carbon dioxide. The hole was cased to 30 m, but the aggressive water quickly attacked the pipe, and remnants of it still rose from the pool after the Second World War. In 1975 the mofette was given a well shaft with a 40 cm gravel layer, and in 1983 it was framed with basalt columns. A full rebuild in 2001 narrowed the shaft and turned it into today's cold-water geyser.

Locals affectionately call this place 'Brubbel,' a name that echoes the bubbling and gurgling sounds of the water. The community has embraced this name as part of their identity and incorporated it into the walking trails and information signs.

Eruptions run about every 35 minutes: a 2–4 m jet for a few seconds, then several minutes of boiling overflow that floods the shaft surround and drains through a masonry channel into the nearby stream. Stand back from the rim during the second phase. The water is very turbid from sediments, and escaping gas carries traces of hydrogen sulfide, noticeable as a rotten-egg smell.

Below the surface, the phenomenon works through a complex interplay: carbon dioxide is pushed up from deeper layers, mixes with water, and creates regular eruption cycles. The spring also contains traces of hydrogen sulfide, which occasionally produces a distinctive odor.