The bottomless pit is a location within the hotel which recurs in multiple TV episodes.
In The Mavysitters Club it is where Mavis said Hank fell, causing Klaus to jump in. Then when Hank actually falls, Pedro falls in after him and Mavis flies them all out.
Mavis mentions this is where the Siren would go to get back to the sub-underworld
In Don't Fear the Realtor they try to fake that it has a bottom to make it less appealing for sale.