Burrow openings are indestructible hives, which spawn Phazon Hoppers. They are only encountered in the Metroid Cavern on Phaaze, where they hang from the ceiling in the second part of the room. Unlike nests of other species of Hopper, Samus cannot destroy these with her weapons. The openings appear to be composed of hardened Phazon.
When Samus destroys the eyeballBlocker and moves past it, she comes across an indestructible Phazon mass in her path. It glows brightly, and these openings follow it, slowly spawning a dozen Phazon Hoppers to attack Samus. Once she has defeated them all, the mass begins to sizzle and then explode, allowing her to continue.
"Burrow opening is made of hardened Phazon and is too strong to be collapsed."