Schematic depiction of a <a href="https://en.wikipedia.org/wiki/Cross-cap" class="extiw" title="en:Cross-cap">cross-cap</a> with an open bottom, showing its <a href="https://en.wikipedia.org/wiki/Level_set" class="extiw" title="en:Level set">level sets</a>
<a href="//commons.wikimedia.org/wiki/User:David_Eppstein" class="mw-redirect" title="User:David Eppstein">David Eppstein</a>
<span class="int-own-work" lang="en">Own work</span>