Depleted Bloontonium Darts, sometimes known as Depleted Bloontonium, is the second upgrade for the second path for the Dartling Gun which allows the tower to pop any bloons, including Lead Bloons. It costs $850 on Easy, $1000 on Medium, $1080 on Hard and $1200 on Impoppable. The upgraded darts that the Dartling Gun shoot appear the same as normal darts, unless in BTD5, where they gain a green wavy border around the projectile. Oddly, a Laser Cannon without this upgrade cannot pop Lead Bloons, but a Ray of Doom can regardless of this upgrade. The Dartling Gun changes its color to purple and it gets a radioactive symbol on the top of the gun when upgraded to Depleted Bloontonium Darts, unless any tier-3 upgrade is bought.
It's useless on the Ray of Doom because they can already pop Lead Bloons. However, most people would normally upgrade to Depleted Bloontonium Darts before the Ray of Doom because Ray of Doom is much too expensive.