Sótano de las Golondrinas

The Cave of Swallows is a limestone shaft that opens in the forest floor as a hole about fifty metres across and drops in free fall for around three hundred and seventy metres on the shallow side and over five hundred on the deep one, wide enough at the bottom to hold most of the world's tall buildings. Its floor is never reached by direct light. Tens of thousands of white-collared swifts and green parakeets roost on the walls, and at first light they leave together, spiralling upward in a widening column that takes half an hour to clear the mouth; at dusk they return by folding their wings and dropping straight in. Cavers rappel the full drop, a descent of several minutes on the rope.

What you actually see

• The dawn exodus - birds spiralling out of the pit in a rising column.
• The dusk return - swifts folding their wings and dropping in.
• The viewing platform - the rim edge above the free-fall drop.
• The rappel - a several-minute descent on rope for experienced cavers.

When to go

October to April is drier and the trails to the rim are less slippery. The exodus starts before sunrise, so arriving in the dark is necessary; the return flight is in the hour before dusk. Bird numbers are lower in the wettest months.

Honest expectations

Seeing the dawn flight means leaving your base around four in the morning and walking a steep stepped path in the dark. The rim has a platform but the drop is unfenced in places and taken far too casually. Bird numbers vary greatly and a quiet morning is possible. Rappelling requires a licensed operator and is not a beginner activity.

Nearby

Tamul waterfall and the Huasteca rivers are within an hour; Ciudad Valles is the regional base.