Mravenčák, Natural monument in Lestkov, Czech Republic.

Mravenčák is a cone-shaped hill and natural monument on the right bank of the Ohře south of Klášterec nad Ohří. Its summit reaches 531 metres, and on the south-western slope a three-to-six-metre rock wall of leucitic tephrite and tuff, known as Jakubova skála or Čertův zub, rises above the protected area of 1.5 hectares.

The hill was long deforested and farmed; remnants of small meadows and old fruit trees survive in the forest. The protected area on the summit was declared on 22 November 1990 by the Chomutov district national committee under registration number 1507. Historical sources also record the hill as Hackelberg, Häckelberg, Hekberg, Kegelberg or Mravenečník.

The name Mravenčák is often linked to mravenec, Czech for ant, but historical sources record the hill as Hackelberg, Häckelberg, Hekberg, Kegelberg or Mravenečník. In the forest around the protected summit, remnants of small meadows and old fruit trees survive as traces of former clearing and farming.

The site is accessible on foot through the mountain landscape and sits within an area of marked hiking trails. Visitors should bring weather-appropriate clothing, as conditions in the mountains can change quickly.

The protected area covers 1.5 hectares between 490 and 531 metres, split almost evenly between forest (0.72 ha) and bare rock (0.78 ha). On the rock ledge of the southwest slope grows the Bohemian pasqueflower (Pulsatilla pratensis subsp. bohemica), the botanical highlight of the site. Wild boar root up the soil and tourists damage the plants, which threatens the steppe flora.