Nesting tunnels for bee-eaters. At least used by the northern carmine bee-eaters and red-throated bee-eaters. Never saw the other two bee-eater species in the hall near the tunnels.
I only saw the red-throated use these on my visit.
And yes I agree, seeing 4 species of bee-eater in one hall is really cool. Managed to find 3 of them quite fast, but it took me quite a while to find the fourth (the carmine).