• Biggest show on rough days, when swells push through the arch and spray rises over the cliff; on calm days the water is much quieter
• Steep drops into rough sea around the rocks, so keep kids on the viewpoints and hold hands near the edges
• Coastal walkway from town links the viewpoint to central Cascais, easy to combine with the seafront and beaches

Boca do Inferno ("Mouth of Hell") is a sea-carved rock formation on the cliffs just west of Cascais town center. Atlantic waves rush through an open arch into a collapsed cave below, booming and throwing up spray, most of all on stormy days.

Viewpoints look straight down into the chasm, and a coastal walkway links it to central Cascais, so it pairs easily with a stroll along the seafront.