Graham Eaton / Underwater view of a fast flowing stream, showing undercut bank on the outside of a small meander, Snowdonia NP, Gwynedd, Wales, UK, February

Graham Eaton / A shoal of Minnows (Phoxinus phoxinus) hiding in an undercut. River Glaslyn, Beddgelert, Wales, May.

Graham Eaton / Underwater view of a fast flowing stream channel, showing undercut bank on the outside of a meander. Snowdonia, Wales, February 2011.

Bryan and Cherry Alexander / Icicles hanging from the edge of an undercut iceberg by the shore of Snow Hill Island, Antarctica, October 2009.

Bryan and Cherry Alexander / Icicles hanging from an undercut iceberg by the shore of Snow Hill Island, Antarctica, October 2006.

Bryan and Cherry Alexander / Icicles hanging from an undercut iceberg by the shore of Snow Hill Island, Antarctica, October 2006.

Bryan and Cherry Alexander / Icicles hanging from an undercut iceberg by the shore of Snow Hill Island, Antarctica, October 2006.

Bryan and Cherry Alexander / Icicles hanging from an undercut iceberg by the shore of Snow Hill Island, Antarctica, October 2006.

Mark Taylor / Ko Tapu limestone pinnacle islet. Phang Nga Bay, Thailand, December 2010.

Mark Taylor / Ko Tapu limestone pinnacle islet. Phang Nga Bay, Thailand, December 2010.

Mark Taylor / Long-tail boat and limestone cliffs. Koh Phi Phi, Thailand, December 2010.

Tim Laman / Limestone uplift islands with edges undercut by the sea, covered in rainforest vegetation, Palau, Micronesia

Juergen Freund / Undercut rock and ledge behind the waterfall at Fern Pool - a permanent swimming hole surrounded by lush vegetation, Karajini National Park, Western Australia

Colin Monteath / Undercut iceberg, Petermann Island, Antarctic Peninsula, Antarctica