Sclera, the protective covering of the eye, is thinnest at the following site:
Apex
Limbus
In front of inseion of rectus muscle
Behind inseion of rectus muscle
The sclera is the fibrous outer coating of the eye, consisting almost entirely of collagen. It is 1 mm thick posteriorly, 0.66 mm at the inseions of the rectus muscles, 0.33 mm beneath the rectus muscles and is thinnest just behind their inseions. Ref: Parsons' Diseases of the Eye By Ramanjit Sihota & Radhika Tandon, 21st edition, Page 218.