Alviela Cave

Alviela Cave (Nascente do Alviela, also called Olhos de Água do Alviela) is the most important karst spring in Portugal, set in the municipality of Alcanena at the foot of the Maciço Calcário Estremenho. The Alviela River rises here from a network of flooded limestone passages. The cave entrance is underwater, so the cave itself belongs to technical divers, while most visitors come for the spring and the riverside setting.

What is Alviela Cave?

The spring emerges at the base of a cliff where the limestones of the Estremenho massif meet the tertiary sandstones of the lower Tagus basin. It drains a karstified catchment of roughly 180 km². Flow is steady and rises noticeably after heavy rain. It ranks among the deepest springs in the world and connects to a cave complex regarded as Portugal's most significant fluvio-karst system.

Why is it special?

• Lisbon's water. The Alviela has supplied the capital since 1880 through the Alviela Aqueduct.
• Bats. Several colonies roost in the underground sections, more than 5,000 animals in all.
• Cave life. In 2007 unpigmented specimens of the invasive red swamp crayfish were recorded in the spring, raising concern for native underground species.
• Diving. Divers have explored the system for years. Showcaves.com lists about 1,250 m of length and 215 m of depth, and expeditions in 2020 pushed further.

What can you see on site?

Beside the spring is the Olhos de Água river beach, with a bar, toilets, a playground, picnic areas and parking. The municipality does not officially classify it as a river beach, so there are no lifeguards. Next door, the Centro de Ciência Viva do Alviela (Carsoscópio) is a paid science centre about karst. Opening hours vary by season and by source, so check before you go.

How do you get there?

The address is Praia Fluvial dos Olhos de Água do Alviela, Louriceira, 2380-450 Alcanena. Approach from the A1 and A23 motorways and finish on a local road toward Amiais de Baixo, with a signposted turn about a kilometre before the village. Coordinates: 39.4456 N, 8.7121 W.

When should you visit?

The spring flows all year. Summer suits picnics and a dip in the river, while after rainy spells the outflow is at its strongest.

Frequently asked questions

Can you enter Alviela Cave? Not without specialist equipment. The entrance is submerged and only trained cave divers explore it.

Is the spring free to visit? Viewing the spring costs nothing. The Ciência Viva centre charges admission.

Can you swim there? The river beach is popular, but with no lifeguards you swim at your own risk.

Where is Alviela Cave? In the municipality of Alcanena in central Portugal, roughly 100 km northeast of Lisbon.