Heaven's Pillar

Heaven's Pillar (そらのはしら Tenshira) was a secret Project by the World Government as a direct means for Humans to reach Skypiea. It was headed by  Shichibuka i, Melchiorra D. Tsubasa who had left Skypiea after the Knock Up Stream was deactivated. The project was ultimately a failure and was shot down, its remains are at the New World of the Grandline and are strictly off limits.

History
Once negotiations began between Humans, the Knock Up Stream was deactivated as a precaussion. Rumors sturred of a takeover, so Tenshi and Zaiel agreed on making any transport between the two worlds impossible. Secretly, the World Government was planning of this very takeover, hoping to absorb Skypiea into their government. So with the help of  Shichibukai; White Angel Tsubasa, the Government illegal set up Project Tenshira, a tower that would transfer anything directly into the sky.

Official workers couldn't be hired to work on the tower, so prisoners and orphans were turned into slaves and forced to work on building the tower. They worked under poor conditions and the project went on for years, ultimately becoming a failure because it could not break the seal set on the sky by the Skypieans. Eventually negotiations were finished and the Knock Up Stream was opened, seemingly ending any threat of an uprising.

Aftermath
Tsubasa came up with a secondary plan after Tenshira's failure, he took a special group of orphans and impanted them with the remains of "God Enel". This transformed them into powerful sky beings known as Birkans. He erased their memories and placed them at the head of an uprising that would succeed in destroying the Skypiean peace and ultimately setting up a government controlled by Tsubasa without anyone elses knowledge.