Undersea corral? Enchanted castles? Space serpents? These eerie, dark pillar-like structures are actually columns of cool interstellar hydrogen gas and dust that are also incubators for new stars. The pillars protrude from the interior wall of a dark molecular cloud like stalagmites from the floor of a cavern. They are part of the "Eagle Nebula" (also called M16 the 16th object in Charles Messier's 18th century catalog of "fuzzy" objects that aren't comets), a nearby star-forming region 6,500 light-years away in the constellation Serpens.
The pillars are in some ways akin to buttes in the desert, where basalt and other dense rock have protected a region from erosion, while the surrounding landscape has been worn away over millennia. In this celestial case, it is especially dense clouds of molecular hydrogen gas (two atoms of hydrogen in each molecule) and dust that have survived longer than their surroundings in the face of a flood of ultraviolet light from hot, massive newborn stars (off the top edge of the picture).
Image credit: NASA, ESA, STScI, J. Hester and P. Scowen (Arizona State University). License: (paraphrased) "Material was created, authored, and/or prepared for NASA under Contract NAS5-26555. No claim to copyright is being asserted by STScI, and material may be freely used in the public domain. It is requested that any subsequent use of this work NASA and STScI be given appropriate acknowledgement. STScI further requests voluntary reporting of all use, derivative creation, and other alteration of this work to firstname.lastname@example.org."