Energon lake

The energon lake is a reservoir of energon located in the Sea of Rust. It's possibly the only major source of energon left on Cybertron.

Fall of Cybertron
Six days before the Ark was launched, the Lightning Strike Coalition left their posts protecting the Ark to investigate strange energy readings in the Sea of Rust. Once there, they discovered that the readings were caused by Shockwave's experiments at the energon lake. Then, they were captured by more of Shockwave's experiments, the Insecticons and became experiments themselves, being given dinosaur alt-modes by Shockwave. Jazz and Cliffjumper later stumbled on the lake when looking for Grimlock's team. After defeating Shockwave, they reported the existence of the lake back to Optimus Prime. The Autobots sent a huge transport to the lake to retrieve the energon so they could use it to power the Ark's launch.