Engineers have described how they had to drain a reservoir to restore a dam dating back to the golden age of slate mining in Gwynedd.
The work was carried out at Hendre Ddu in the Dyfi Forest, between Dolgellau and Machynlleth, after the reservoir was unfit to cope with flood risks.
Engineers say they were "dealing with the unknown" as they built a ramp to the bottom.
More than 200 tons of solid rock were also removed over a month.
Water from the reservoir at Hendre Ddu was used to power machinery in slate cutting sheds in the 1860s.
There are several old slate mines in the area around Abergynolwyn, Corris and Aberllefenni.
The reservoir enabled Sir Edmund Buckley to establish the Hendre Ddu Slate and Slate Company in 1864, which specialised in producing slate slabs for billiard table beds.
Dave Farmery, the local area manager with Forestry Commission Wales, said it had two options.
They were either to strengthen the reservoir, which was built over 100 years ago, "for historical and conservation interest", or to breach the dam.
"Although this is not a designated monument, the site is of local interest, " he said.
"We were determined to come up with a design which made the dam safe, protected the water quality, and at the same time preserved this special landscape character within the Dyfi forest," he added.
Engineers - helped by three different contractors - increased the capacity of the dam wall by one metre, and increased the volume of the spillway so that more water could run off the reservoir.
Mark Trumper, the Forestry Commission's Wales area engineer, said it was a challenging project working in the murky depths of the reservoir, which is between 10m - 15m (about 33ft - 50ft) deep.
It took two weeks to drain the water.
"We built an access ramp down to the reservoir as the water level went down, to enable us to get to the bottom of the lake," said Mr Trumper.
"In effect, we built a road to the bottom of the lake. It was like a sludge pond - we were constantly dealing with the unknown," he said.
An additional challenge was to locate a pipe in the sludge and fit a plastic filtration elbow on an existing Victorian structure.
The weather stayed fine however which helped, he added.
"We had a lot of rain beforehand and afterwards, but if it had rained hard during the job, I'm not sure if the pumps would have coped," he said.
"Things were constantly changing, not by the day, but by the hour.
"It was the most challenging job I've taken on," Mr Trumper added.