The mole rats hollowed out the block of concrete. By biting it.
This is possible because those teeth are hard as hell, and also 25 percent of the animal's muscles are concentrated in its jaw (in humans, it's only 1 percent). Also, one-third of their brain cortex is devoted to processing information from the mouth area alone. It's like evolution completely gave up on the rest of their bodies and focused on making their mouths as badass as possible. If all their organs were as deadly, they would be ruling the planet by now.
If they secretly aren't already, from underneath.
They can even move the top teeth independently, like chopsticks, which they use to dig vast underground tunnels and retrieve buried food. Sure, they still look like morons, but we wouldn't say that to their faces.