Delos holds two facts that sit within a few hundred metres of each other and are almost never stated together.
The purification. The island was held to be the birthplace of Apollo and Artemis, and its sanctity was enforced by legislation of a kind with few parallels. In 426 BC Athens purified Delos: every grave on the island was dug up and the remains reinterred on neighbouring Rheneia, and thereafter no one was permitted to be born or to die there. Women approaching labour and people who were seriously ill were ferried across the strait. The island was to contain no birth and no death because the god had been born there.
This was not purely piety. Athens controlled the sanctuary and the treasury of the Delian League was kept there until it was moved to the Acropolis, and a purification asserted Athenian authority over the island in the most visible way available.
The market. In the second and first centuries BC, after Rome made it a free port, Delos became one of the largest slave markets in the Mediterranean. Ancient writers report, with the usual caution owed to round numbers, that many thousands could change hands there in a day. The same small island that no one was permitted to die on was a clearing-house for human beings.
There is no need to editorialise about this and this entry does not; the juxtaposition is simply a fact about how ancient sanctity worked, and about how far ritual purity and moral concern were separate categories. Pollution was a technical matter concerning birth, death and blood. Selling people was not pollution.
Delos was sacked twice in the first century BC and never recovered. Nobody lives there now. The French School at Athens has excavated since 1873 and uncovered an unusually complete Hellenistic town alongside the sanctuaries: houses with mosaics in place, and shrines to Egyptian and Syrian gods raised by resident merchant communities, which makes the island one of the best records anywhere of ordinary Mediterranean religious mixture.
Its problem now is the sea. Low-lying, unprotected and fully exposed, it is losing ground to storms and rising water.