Blaue Quelle, Natural spring in Erl, Austria.

The Blaue Quelle is a karst spring at 471 m above sea level, about 300 m southeast of Erl's Passionsspielhaus. It feeds a pond whose clear, deep-blue colour gave the spring its name; the sandy bottom is covered with stoneworts. Chestnuts, beeches, oaks, ashes, lindens and birches surround the pond. The water drains via the Mühlgrabenbach and Trockenbach into the Inn after roughly one kilometre.

In 1926 the spring, its pond and the surrounding group of trees became the first object in Tyrol to be declared a natural monument. The site is now in private ownership, and access for visitors is no longer permitted.

The name Blaue Quelle comes from the deep blue colour of the spring pond it feeds. The pond's sandy bottom is covered with stoneworts, and the water is ringed by a group of horse chestnuts, beeches, oaks, ashes, lindens and birches. Spring, pond and trees were together declared Tyrol's first natural monument in 1926.

The spring lies about 300 m southeast of the Erl Passion Play House at 471 m above sea level. It is now private property and visitors are no longer allowed access, so the pond and its surrounding trees can only be viewed from outside.

The spring's temperature stays almost constant at 8.1 °C, varying by only 0.2 °C, and its conductivity ranges narrowly between 516 and 596 μS/cm. Hydrologists read this steadiness as a sign that much of the water comes from deep karst. Since 1992 a gauging station (number 395434) of the Tyrolean Hydrographic Service has recorded the flow.