Gōngchéngshī men máitóu-kǔgàn, zhōngyú gōngkè le nántí.
The engineers toiled head-down and finally cracked the problem.
Tips
memory
埋头 literally = 'bury your head' - picture someone bent over work so intently they never look up. 苦干 = 'bitter grind'. The image is total focused effort without fanfare.
