Boca do Inferno

The Boca do Inferno is a sea cave in the limestone west of Cascais whose roof has fallen in, leaving a chasm connected to the open Atlantic by an arch. On a calm summer day it is a hole in the rock with green water sloshing through it and people photographing themselves at the railing. In a winter storm it is the reason for the name: swell funnels through the arch and erupts up the shaft, throwing spray over the cliff and the road, with a boom you feel through the ground. Walkways and railings run around the rim. The site has a strange footnote - in 1930 Aleister Crowley faked his own death here with a note left on the rocks, a stunt arranged with the Portuguese poet Fernando Pessoa, and reappeared in Berlin three weeks later. There are stalls and a café at the top.

What you actually see

• The collapsed cave - a chasm linked to the sea by a rock arch.
• The winter surge - swell erupting up the shaft and over the cliff.
• The walkways - railed paths around the rim of the hole.
• The 1930 hoax - Crowley's staged death, arranged with Pessoa.

When to go

October to March for the dramatic version; summer is calm and much less impressive. Thirty to sixty minutes. Time it with a storm swell for the full effect.

Honest expectations

In calm weather it underwhelms - it is simply a hole in the rock. In storms the cliff edge is dangerous and people have been swept off; stay behind railings. Crowded with coach stops in season. Persistent souvenir stalls.

Nearby

Cascais is east along the coast road; Guincho and the dunes are west.