be hard at work(+ gerund; example provided by ART Vancouver • Our design and restoration team has been hard at work bringing the 16th-century castle's interiors back to life.)
put one's nose to the grindstone(The mayor should be putting his nose to the grindstone to fix this problem right now.ART Vancouver); keep one's nose to the grindstone(to do hard, continuous work (Merriam-Webster) | to work very hard for a long time (Cambridge Dictionary) • It's 11:37 am...just about finished their 2 hour break in time for their 3 hour lunch. Then nose to the grindstone for 15 minutes of solid work and call it a day. Brutal working conditions...ART Vancouver)