The phrase "dig hard" is correct and usable in written English.
You can use it to imply that one should put a lot of effort into a task. For example: "If you want to succeed, you'll have to dig hard."
If we dig hard, we might find who and what influenced all three, but in essence these are men who arrived with something never heard before.
Wait long enough and dig hard enough and you'll run across an athletic feat that is both awe-inspiring and emotional, like Laura Wilkinson's unexpected gold-medal dive.
Introducing a discussion about Leveson's response to the representation of women, Woman's Hour presenter Jenni Murray pointed out: "You have to dig hard to find it".
They fear that corporations may hide the monitors' findings from the public, and that the accounting firms doing the monitoring will not dig hard enough to find problems.
The colleague in question was working at The Spectator during Boris's editing days, and instructed by his blond boss to dig hard into a 'report' that Tony Blair had tried cannabis in his gap year before Oxford.
Ha ha ha! Halloween costumes are a minefield of potentially offensive images if you want to dig hard enough, but then ghosts or cowboys don't make up a big chunk of modern society.
"We've got to dig harder," Mr. Knudson said, suddenly resolved, his voice booming into the night air.
Some scientists say that the molecular data are likely to be right and that paleontologists need to dig harder for those fossils.
And while the filmmakers may believe that by not taking a strong position, they're giving viewers room to make up their own minds, they would need to dig harder to inspire that kind of engagement.
"When I didn't find it, I thought, 'I'll have to dig harder.'".
"Papa," he said, dug hard with his fingernail.
