"Now, sources within the movement have told local media that the tunnels are linked to fringe beliefs that a dead former leader is the Messiah, who arrived on Earth to usher in the End of Days, and is secretly still alive."
"A source told The New York Times that those behind the tunnel’s construction were building it in order to “hasten” expansions into nearby buildings.
That group, known as Messianic Chasidim, believe Rebbe is the Messiah and is in fact still alive, and view the site as their Temple."
https://www.news.com.au/lifestyle/real-life/news-life/for-the-messiah-bizarre-new-details-about-why-tunnel-system-was-secretly-built-beneath-new-york-synagogue/news-story/37f4771256f5581001c9668fb98305d9
"The other group, which includes most of Crown Heights’ estimated 15,000 Chabad adherents plus others, especially in Israel. This group worships in the basement synagogue and is sometimes called Congregation Lubavitch. Members of the congregation believe Schneerson never died. He is in hiding, possibly in the building, and could re-emerge at any moment.
“They believe if they could expand the synagogue, it would sort of help usher in the reappearance of the rebbe, the Messianic Age, the end of times,” said Rabbi Yossi Newfield, a former Chabad member and a scholar whose family members still live in the community."
https://religionnews.com/2024/01/18/how-a-battle-over-a-legacy-of-chabads-late-rebbe-led-to-a-brawl-over-an-underground-tunnel/