I think it was some teleport door. Or not, I am not sure.
Anyways, I went down there through that super long section with the upwards gravity fields.
where at the beginning, I basically had to swing uo at the highest point to just barely fall far enough through the gravity area to thing below.
anyways , made my way down there.
also got the yellow(?) key down there and being going like to the right and always downwards, I kind of made it to the mole village.
not sure about any specifics anymore, getting through all o that without dying wasnt exactly easy.
especially with the invincible fire balls circling along the edges of platforms.