He also created an air sculpture out of one thousand and one helium filled blue balloons,
which he then released into the sky, thus making them defy gravity and merge with the vast open space of the sky. This performance took place Saint-Germain-des-Pres in 1957. In the same year he came up with the idea of the Blue Obelisk. He would have like to have illuminated the Obelisk in the Place de la Concorde in Paris with blue light in hopes of creating the effect of the Obelisk floating above the ground. He wanted something which was very heavy to create the illusion that it was defying the laws of gravity.

     Still not satisfied that he had captured the thoughts and concepts of the air sculpture, the idea of the sponge sculptures emerged, and Klein reconceptualized the air sculptures into ìaerostatic objects". He made hundreds of blue sponge sculptures of varying heights, shapes, sizes and textures, which were put on display on June 15,1959, at the Galerie Iris Clert.  He displayed the sculptures on the back and side areas of a small room, ìtransforming the gallery into a lush, crowded, sylvan environment.î  (Stich, p.161). This project was meant to allude to the beauty and overgrowth of a natural woodland area. 

     To create the sponge sculptures, he would place one sponge, or a cluster of sponges, on top of a metal rod which was ìaffixed to stone bases or metal plates or grounded by twisting the rod itself into a footingÖ The rods were diversely tall & short, curving & straight; & the bases ranged from giant or small rooks with craggy or smooth surfaces to flat, shiny, or swirling footings." (Stich, p.161). Since no sponge sculpture was ever really the same, the gallery space was filled with an assortment of things which resembled flowers, trees, strange vegetation, and even human figures (Klein often referred to his sponge sculptures as "portraitsî)

     The forest of sponges is meant to imply that ìa process of natural growth and proliferation was taking place" (Stich, p.165), that these blue trees and flowers were natural growth In a small area which were evolving and multiplying as Klein continued to create hundreds of blue Standing sculptures and ìunattachedî blue sponges. The forest was meant to emphasize an allusion to nature, and reference the differences which can be found within a seemingly similar object.  He was interested in the diversity which occurred in multiplicity. Along with these mentioned standing sponge projects, Klein created sponge relief paintings (a blue monochrome painting with blue sponges attached to the surface). These painting were exhibited with the standing and attached sponge sculptures, adding to the diverse appearance of each sponge.
Klein became obsessed with creating sponge based works due to the fact that he could ìproduce quantities of coloured artifacts and do so without adopting the mechanical means of mass production he so vehemently condemned and without eliminating the variability factor he so favoured.î (Stich, p.165)  The sculptures were trying to convey a sense of return to nature, a sense of freedom, of having not set boundaries, of being able to defy gravity's pull.  These works emphasis that no two things on the planet are exactly identical despite their apparent similarities.

     Since sponges were naturally absorbent, they were the perfect materials to use to provide Klein with ìthe kind of saturation he sought to achieve:  Saturation of both the environment and the peopleî (Stich, p. 165).  He saw his sponge projects as perfect examples of successful ìimpregnation process [which] held their own as suggestive, indefinable, ambient, penetrative phenomenaî (Stich, p. 165).  Klein wanted his audience to fell that art was still connected to the experience of real life, explaining his desires to create sculptures which reference the natural world.  He hoped to offer a piece of freedom, of weightlessness, served along side a slice of reality, thus making the audience feel weightless, yet grounded, just like the sponge sculptures which were trying to brake free and float into space.

     His sponge air sculptures are a natural progression in Klein's work as he searched for a form of freedom from weightiness, a defiance of gravity.  It is an obvious concept which reappears through out his career in such works as The Void (1958, where the viewer enters an empty room and is surrounded by nothingness, by empty space), and Leap into the Void (1960, appearing in Dimanche with the caption ìUn  Homme dans l'espace! Le peintre de l'espace se jette dans le vide!Ö.Obsession de la levitation.î (Stich, p.216).  He also made a work where he signed the sky with his name, in a sense making him become vicariously weightless and in the sky since he is identified and recognized as being Yves Klein.  The theme of levitating into open space is unmistakably a reoccurring, driving passion behind most of the works created by Yves Klein.