洞 is a Japanese kanji written with 9 strokes, listed here under the keyword “den”. It is built from 水 (water) and 同 (same). In Remembering the Kanji by James W. Heisig it is frame 193.
On KumiJi Glyph Explorer, open the parts search and type “water” + “same”. 洞 is among the results, together with every other character containing the same shapes.
Open 洞 in the interactive kanji search
to expand every part down to its smallest pieces, add your own names and mnemonic stories,
or search for other characters by their parts.