Remember the Item Sphere the Diggernaut stole earlier? Once the boss is defeated, the Item Sphere drops out and is yours to claim. Inside is the Metroid series staple: Power Bombs.
Roll directly under the hole in the ceiling, Spider Ball to grip the floor, then lay a Power Bomb.
The Diggernaut’s shell blocks your way out of the room. Plant a Power Bomb to destroy its remains and gain access to the leftmost door.
Voilà! An easily obtained Power Bomb via extremely impressive means. Once you grab the Power Bomb Tank, drop down and Space Jump onto the platform left of the Yellow Door. To open it, plant another Power Bomb.
Jump into the Morph Ball passage and roll to the right. When you reach a dead end, plant a Power Bomb to destroy a Power Bomb Block overhead.
Take the door on the right side of the room, then drop into the opening ahead. Pull the Pull Block you find there.
Bomb the Blocks near the left wall and drop down the opening. Another Pull Block obstructs the way.
After you remove the Pull Block, roll through the Morph Ball passage and jump onto the platform above and to the right. From here it’s a straight shot to the elevator, so make haste!