This lake is not a natural formation. Keris is sure of it. They dug it out. The centre of the lake simply drops off, in a nearly vertical fall which goes down four, six, eight hundred metres. The low bit is maybe fifty metres wide, but... no wonder they made a hillock here. There used to be a barrier perhaps twenty metres from the top, but it's broken. Its debris litters the bottom, along with hundreds of years of sediment.
And the bottom isn't the bottom. Oh, it's a bottom. A bottom which is tens of metres thick. But when she swims deep, deep, deep down - so deep there's no light at all and puts her ear against the bottom, she can hear the things in the white stone at the bottom, below the thick sediment. The hissing things in metal cylinders. The sound of water boiling. The clattering and clanking.