[{"id":0,"comments":[],depth:0},{"id":1,"comments":[{"id":3,"comments":[{"id":5,"comments":[],depth:2},{"id":6,"comments":[],depth:0}],depth:1}],depth:0},{"id":2,"comments":[{"id":4,"comments":[],depth:1}],depth:0}]

db.collection.update({"comments.comments.id":5},{"$push":{"comments.$.comments.$[elem].comments":{id:"newId",depth:3,comments:[]}}},{arrayFilters:[{"elem.id":5}]})