It is 100% true that the dead body of Prophet Muhammad (peace be upon him) is in fresh and healthy condition without chemicals in his grave under the Earth since 1400 years in “Madina” which is a city of Islamic country Saudi Arabia.
The Claim:
Prophet Muhammad (peace be upon him) claimed that he is the last messenger of God and the Holy Quran has revealed to him through an Angel named Jibril in 23 years.
The Proof:
Fresh dead body of Prophet Muhammad (peace be upon him) in healthy condition proves that his claim was correct and he is a true messenger of Almighty Allah otherwise his dead body could have been decayed like other humans because dead bodies of all corrupt Muslims, non-Muslims and Animals decay after death and the Earth does not have capacity to protect any dead body in it so we should accept that this protection is by the God, and he is happy from him.
Non-Muslims Plan to Kidnap Fresh Dead Body of Prophet Muhammad:
In 1162 A.C, two Non-Muslims were reaching to his grave by digging through underground passage to take out his dead body, but the Holy Prophet Muhammad (peace be upon him) spiritually informed present king of Saudi Arabia named "Nooruddin Zangi" via dream and showed their pictures too. After this, they were checked by king and caught. Moreover, under his orders, a channel dug around the Prophet's grave and filled with molten copper to protect his grave from any further mischievous attempts forever.
This also proves that those Non-Muslims knew that Prophet Muhammad (peace be upon him) is true and his dead body is in fresh condition therefore they tried to take out it.
Non-Muslims should ask about this shocking news from their top religious and spiritual leaders, they will attest this reality unless they want to hide truth.
Muslims accept this truth because it is mentioned in Hadith of Abudaood and Nisai that everyone can see Jamul Fawaid Volume no 1 Hadith no 1924.
Conclusion:
So in the light of above reality and truthfulness, Non-Muslims should accept his claim and enter in religion Islam so creator of this universe will be happy from them.