YOU SAID:

 Fire often smolders underground for days, then flames again.

INTO JAPANESE

火は地下で数日間くすぶり、その後再び炎上することがよくあります。

BACK INTO ENGLISH

Fires often smolder underground for several days and then flare up again.

INTO JAPANESE

火災は地下で数日間くすぶり、その後再び燃え上がることがよくあります。

BACK INTO ENGLISH

Fires often smolder underground for several days before reigniting.

INTO JAPANESE

火災は、再燃するまで数日間地下でくすぶることがよくあります。

BACK INTO ENGLISH

Fires often smolder underground for several days before reigniting.