Tawi Attir (the hole of the birds), is a natural sink hole over 100 metres wide and 250 metres deep. It is the second deepest sinkhole in the world. As the name implies, Tawi Attir is home to many species of birds that can be seen swooping in and out of the sinkhole during khareef (monsoon) season.
Tawi Atayr cross section taken from Hanna & Al Balushi, 1996. Note the schematic path at its western side.
Useful links : http://home.kpn.nl/janm_schreurs/Tawiattair.htm