For that reason, the Prophet ﷺ informed us that whoever usurps even a small or large piece of land shall come on the Day of Resurrection under the severest punishment: his neck will swell and lengthen, and the very land he seized—together with everything beneath it down to seven earths—will be fastened around his neck in retribution for having wronged its owner by taking it.

Points derived from the ḥadīth
1- Usurpation is forbidden, for it is a form of injustice which Allah has forbidden for Himself and has likewise made unlawful among us.
2- Oppression is prohibited whether in small or great matters; this is the benefit of the Prophet’s mention of “a hand-span.”
3- Real estate is deemed usurped by mere physical occupation and seizure. Al-Qurṭubī said: the ḥadīth shows that land can indeed be usurped and that doing so is one of the major sins.
4- Whoever owns the surface of a plot also owns what lies beneath it all the way to its depths. Therefore no one may dig underneath it, construct a tunnel or passage, and the like except with his permission. He likewise owns any buried stones or minerals and may excavate them at will. Scholars have further said that the airspace follows the land, so whoever owns a tract owns what is above it as well.
5- Shaykh al-Islām Ibn Taymiyyah said: If illicit wealth—such as property taken by usurpation, money from usury, or gains from gambling—is mixed with lawful wealth so that the two become indistinguishable, the mere mingling is not forbidden. Thus, if one knows that some such wealth exists in a town yet cannot identify it, it is not unlawful for people to buy goods in that town. However, if a man’s wealth is mostly illicit, is it forbidden to deal with him, or merely disliked? There are two scholarly opinions. If most of his wealth is lawful, transacting with him is not forbidden.
6- He also said: According to the majority of scholars, when the owner of a sum of money cannot be identified, it should be spent for the communal benefit of the Muslims. Therefore, if someone holds usurped goods, borrowed items, deposits, or pledged property and has lost all hope of locating the owners, he should give them in charity on their behalf, direct them to public welfare projects, or hand them to a trustworthy person to do so.

Additional note
Al-Mughnī states: “Whatever lies in public streets, roads, and open squares within inhabited areas may not be appropriated by anyone—whether the area is wide or narrow, and whether that appropriation would inconvenience people or not—because all Muslims share in it and their welfare depends on it; it is therefore akin to their mosques. One may, however, make ordinary use of it by sitting there.”