Belmont Island is a tiny 20,000 square foot artificial island just south from Roosevelt Island and to the east of the United Nations. The island was created during the construction of trolly tunnels dug under the river in the 1890s. A shaft to the tunnels dug through the granite of Man-o'-War reef and the excess material was piled on the reef to create this islet. The island is named after August Belmont, the financier that completed the tunnel project after original builder William Steinway died during construction. The tunnels later became part of the NYC subway system, which Belmont also initially financed.
In 1977, the islet was leased from NY State by employees of the United Nations, who landscaped the surface and unoficially renamed it after Burmese Buddhist United Nations Secretary General U Thant. The islet became a protected sanctuary for migrating birds and public access is limited.