This breeding took place in an aquarium, but it had some advantages in its favor. Like the water source was the animals' own native cave water, and the temperature was controlled by being in the cave itself. Still, pretty neat. Photos of eggs and video of olm laying eggs at the bottom of the page:
Rare salamander lays eggs in Slovenia cave | Earth | EarthSky
Rare salamander lays eggs in Slovenia cave | Earth | EarthSky