Nat­ur­al caves don’t come much larg­er than Sơn Đoòng, close to the bor­der between Laos and Viet­nam. This mam­moth abyss has the largest cross-sec­­tion of any known cave on the plan­et, a vast area that is dif­fi­cult to describe. Sup­pos­ed­ly, a Boe­ing 747 could fly through with­out dam­ag­ing its wings, but that doesn’t real­ly do jus­tice to the vast­ness of Sơn Đoòng. The sta­lag­mites here are pret­ty mas­sive too, with some reach­ing up to a whop­ping 70 metres. The jun­gle found inside the cave formed under a col­lapsed roof in one of the caves. After the roof col­lapsed, enough light was allowed into the cave for veg­e­ta­tion to slow­ly creep in from the out­side. As the veg­e­ta­tion gained a foothold, larg­er and larg­er plants began to grow, and now horn­bills, fly­ing fox­es, and mon­keys live in its branch­es.