3 Milhas
Also known as: Marca das Três Milhas
A rock split by a swim-through arch off Sesimbra, reported 8 to 29 m deep.
Last updated September 2026
The dive
A single rock stands split by a fissure that reads from the surface as two separate boulders, with an arc-shaped opening beneath it, wide enough to swim through from one side to the other. Boulders and sandy patches make up the surrounding bottom.
What makes it special
A single passage cuts clean through the split rock, open enough to swim its full length in one continuous pass. The park authority's own coastal dive-site map marks the spot Marca das Três Milhas, an official name for a rock most divers simply call 3 Milhas.
Know before you go
Depth reports for the rock vary widely, from 8-12 m at the shallow end to a maximum of 29 m at the deep end, a gap wide enough to change the certification this dive calls for; book it holding Advanced Open Water, the level the deeper reported figure calls for. Current and surge inside the swim-through, and the best time of year to dive it, are unknown for this specific rock; treat it like any unfamiliar passage and swim through only once you can see it is clear.
Why Dive 3 Milhas
What makes this dive site stand out.
- Split-rock swim-through arch
A single rock split by a fissure, with an arc-shaped gap beneath it to pass through
- Named on the park's dive map
Marked on the Marine Park's own coastal chart as Marca das Três Milhas
Depth & Profile
Conditions & difficulty
The shallower reading needs only Open Water; treat the site as Advanced Open Water because the deeper reported figure cannot be ruled out
The main variable here is depth: a reading that ranges from a gentle 8-12 m to a demanding 29 m for the same rock
Rules and permits
Frequently Asked Questions
Why is Advanced Open Water recommended for 3 Milhas when reports put it as shallow as 8 m?▾
Depth for this rock is reported anywhere from 8-12 m to a maximum of 29 m, a gap wide enough to change what certification the dive needs. Since the deeper figure cannot be ruled out, Advanced Open Water, which covers dives to 30 m, is the safer minimum; a diver confident of staying in the shallow band needs only Open Water.
What does the name '3 Milhas' mean?▾
No documented origin survives for the name. 'Marca' means mark in Portuguese, which hints at a navigational reference point for boats, but no record confirms that reading.
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