Death Mountain's Secret
Go get the Drillshaft behind some destructible rocks nearby. Head toward Bridge of Eldin to the point where small lava falls overhang the path. Climb on the rocks above the lava falls and search for the destructible rocks on the opposite side of the peak from the hot springs. The Drillshaft is behind the wall.
Return to Dugby (can use objective marker) to complete the quest.
Tip: If you have done the Slated for Upgrades Side Quest and have the Sheikah Sensor +, and already have a picture of a Drillshaft, you can set it as your Sensor+ target for help in finding the hidden one. It is #294 for the Sensor+ in the base game, #298 with the Breath of the Wild Expansion Pass and #303 in Master Mode.
Bonus Tip: If you do not already have a picture of a Drillshaft, and if you have done the Sunshroom Sensing Side Quest as well, you can talk to Symin at the Hateno Ancient Tech Lab and purchase equipment pictures for your compendium (at 100 rupees each, getting a random picture each time, can buy in bulk if desired) until you have a picture of a Drillshaft. You can then set it as your Sensor+ target for help in finding one.
Confused Reader Tip: The hidden Drillshaft will not appear on the Sensor+ until the rocks covering it are destroyed.
Additional tip: grab a sledgehammer and you can destroy the wall without bombs.
Dugby, the young Goron soaking on the Goron Hot Springs, said:
"I bid a tweshure binteen gear an ba Bridge of Eldin! Iz an abazing schtick! Beely bool! If boo bind it...iz boars!"
It sounds like gibberish, but maybe there's something to it after all?
If you catch him sleeping on the side of the spring:
"The treasure...at the top of a lava waterfall..."
You found a drillshaft buried in a crag between the Bridge of Eldin and Goron Hot Springs.
Could this be the treasure that Dugby mentioned? Try showing Dugby the drillshaft you've acquired.
The treasure that young Goron Dugby hid and alluded to turned out to be...a drillshaft!
Since you found it, he's going to let you keep it.
Using the Sensor+ does not work for finding this Drillshaft, as it is hidden behind a cracked wall. Once the wall has been destroyed, the Drillshaft can be tracked.