KICKING off a tour of the United Kingdom’s four increasingly disunited nations ahead of Brexit negotiations, Theresa May arrived in Swansea on March 20th bearing gifts. The prime minister announced that the Welsh and British governments would together invest £241m ($300m) in a regional plan to put Wales at the “forefront of science and innovation”. Boosters claim this could be a “transformative” deal for Wales, which has long suffered from industrial decline.

But among the promises, there was one omission: no mention of the Swansea Bay tidal lagoon, a pilot project for a new method of generating electricity. When pressed, Mrs May said that officials were still looking at the idea. Local politicians and manufacturers expressed their disappointment at more procrastination. The government, though, worries about the price tag.

If it went ahead, the project could be the first of its kind in the world (there is another tentative tidal-power proposal in the Bay of Fundy in Nova Scotia). A tidal lagoon works by using the rise and fall of the tides to generate electricity. At Swansea, a 10km (6 mile) seawall would capture the water created by the high tide, which would then be released to drive 16 turbines embedded in the wall. The company behind the proposal, Tidal Lagoon Power (TLP), selected Swansea Bay because it has the second-highest tidal reach in the world, after Nova Scotia, and a shallow seabed.

TLP has lined up the required £1.3bn of private finance, a manufacturing supply chain and cross-party support in the Welsh Assembly, which likes the look of the jobs that it could bring. A report on the potential of tidal power commissioned by the previous government and chaired by Charles Hendry, a former energy minister, endorsed the idea last December. But before it can get under way, Mrs May must sign a deal to buy its electricity.

Attempts to make marine power—that is, wave and tidal energy—commercially viable have lagged behind other renewables such as wind. A few tidal barrages, built across rivers or estuaries, have been operating for years at various sites around the world. But they have never been widely deployed because of the disruption they cause to shipping and the damage they do to the environment. Proposals for a giant barrage across the River Severn, in south-west England, were shelved in 2010. Backers of tidal lagoons say that because they stretch out into the open sea, they interfere less with shipping and bird life—though Swansea’s anglers fret that the turbines would turn their salmon into pâté.

Much of the pioneering work on tidal power has been done at the European Marine Energy Centre in the Orkney islands. Neil Kermode, its director, says his team has tested 17 variations of tide and wave technology from nine different countries. With a technological lead, and big tidal ranges all around its coast, Britain is well placed to exploit tidal energy, argues the Hendry report.

If the Swansea Bay project were to work, Mark Shorrock, the boss of TLP, would like to build five much bigger tidal lagoons at Cardiff, Colwyn Bay, Newport, West Cumbria and Bridgwater Bay. The Cardiff lagoon would have up to 110 turbines and cost more like £8bn. Together they could meet about 8% of Britain’s electricity needs, TLP says. For comparison, the government recently gave the go ahead to the Hinkley Point nuclear power station in Somerset, which is expected to provide 7% of Britain’s electricity. Lagoons have a longer operating life (120 years, against 35 or so for nuclear), are safer and do not have expensive decommissioning costs.

The problem is the price of their electricity. Tidal lagoon power is currently much more expensive than either nuclear or offshore wind. The so-called strike price that the government would have to pay for Swansea’s electricity, to get the project off the ground, would be about £123 per megawatt hour, compared with £92.50 for Hinkley and under £100 for offshore wind. The Hendry report argues that the price of tidal power should fall sharply over the medium term if the technology works and more lagoons are built. That has been the story of offshore wind. The strike price for Hinkley, by contrast, is guaranteed for 35 years.

The government’s hesitation over the initial strike price for tidal power is understandable. And tidal energy is intermittent, since the tides come and go only twice a day. But the Swansea pilot would be a chance to make a smallish bet on a new technology that may yet turn out to be a useful part of Britain’s future energy mix. It looks worth a modest gamble.