Here’s a paragraph from the Clwyd Powys Archaeological Trust Web site about Carn Gop, highlighting it’s Silbury like qualities:
The largest prehistoric monument in Wales is Gop Cairn, near Trelawnyd, Flintshire. The site measures 100m x 68m and is 12m high, and is of mostly stone construction. The cairn was partially excavated by Boyd Dawkins in 1886-7, when a shaft was dug in the centre down to the original ground level. However, despite these efforts no burials or finds were recovered.