According to the Qur'an (18:86), a man named Dhul-Qarnain reached the place where the sun sets. When he got there, he found the sun going down into a muddy pool. Muslim apologists commonly reinterpret the passage, but only because they're ignoring Muhammad's commentary.

Since Muslims in the West claim that science confirms the Qur'an, we should work diligently to explain to them what the Qur'an says, and how Muhammad obviously interpreted it. The following two quotations, when used together, are devastating and unanswerable:

Qur’an 18:83-86—And they ask you about Dhul-Qarnain. Say: “I shall recite to you something of his story.” Verily, We established him in the earth, and We gave him the means of everything. So he followed a way. Until, when he reached the setting place of the sun, he found it setting in a spring of black muddy (or hot) water. And he found near it a people.

Sunan Abu Dawud 3991—Abu Dharr said: I was sitting behind the Apostle of Allah who was riding a donkey while the sun was setting. He asked: Do you know where this sets? I replied: Allah and his Apostle know best. He said: It sets in a spring of warm water.

For more information about Islam, visit Answering Muslims.