diff --git a/surface/GasVentField.gd b/surface/GasVentField.gd
index 29f2f82..e2476ce 100644
--- a/surface/GasVentField.gd
+++ b/surface/GasVentField.gd
@@ -13,6 +13,14 @@ var vents: Array[Vector2] = []
 var progress: float = 0.0
 var emission_phase: float = 0.0
 func _ready() -> void:
+	# #91: the vents are ground features and must never cover the fleet.
+	# SurfaceFeatures is added AFTER Fleet in PlanetSurface.tscn, so at the
+	# default z_index of 0 the later sibling simply wins and the player flies
+	# UNDER their own gas source. Same fix, and the same reason, as #50's
+	# capture rings and the rocks on a rocky surface (Rock.gd) - the
+	# difference is that this field draws in its own _draw() rather than
+	# spawning child nodes, so the depth belongs on the field itself.
+	z_index = -1
 	var rng := RandomNumberGenerator.new()
 	rng.seed = hash(GameState.planet_id if GameState.planet_id != "" else GameState.planet_name)
 	for i in range(6): vents.append(Vector2(rng.randf_range(-800,800), rng.randf_range(-650,650)))
diff --git a/tests/test_surface_feature_depth.gd b/tests/test_surface_feature_depth.gd
new file mode 100644
index 0000000..c6f2af3
--- /dev/null
+++ b/tests/test_surface_feature_depth.gd
@@ -0,0 +1,133 @@
+extends SceneTree
+
+# Headless test: run with
+#   godot --headless -s res://tests/test_surface_feature_depth.gd
+# Forgejo #91: on a gas planet the gas source was drawn ON TOP of the player.
+#
+# SurfaceFeatures is added after Fleet in PlanetSurface.tscn, so anything
+# down there that keeps the default z_index of 0 wins the tie against the
+# ships purely by being the later sibling. The player then flies under their
+# own scenery, which reads as a rendering fault rather than as depth.
+#
+# Checks the RULE rather than the one node that was wrong: every feature that
+# actually draws, on every surface profile that has features, must end up
+# behind the fleet. A feature added later is covered without editing this.
+#
+# Covers:
+#   A. the gas giant's vent field draws behind the ships (the #91 report)
+#   B. the same for every other drawing feature on every profile, rocks and
+#      capture rings included
+#   C. the fleet's own ships are where the comparison says they are, so a
+#      future change that pushed the ships back instead cannot make A and B
+#      pass by moving the goalposts
+
+const PROFILE_PLANETS: Array = ["gas_giant", "rocky", "research_lab"]
+
+var game_state: Node
+var surface: Node
+var planets: Array = []
+var checked: int = 0
+var stage: int = 0
+var elapsed: float = 0.0
+
+
+func _initialize() -> void:
+	# The autoload is reached by node, not by the bare `GameState` name: from
+	# a SceneTree script that name resolves only far enough for constants,
+	# and an instance member through it is a COMPILE error for the whole
+	# file. Every other test here does the same.
+	game_state = root.get_node("GameState")
+	planets = PROFILE_PLANETS.duplicate()
+	game_state.reset_progression()
+	game_state.surviving_ship_count = 3
+	_load_next()
+
+
+func _load_next() -> void:
+	if surface != null and is_instance_valid(surface):
+		surface.queue_free()
+		surface = null
+	if planets.is_empty():
+		return
+	var definition_id: String = String(planets.pop_front())
+	game_state.planet_definition_id = definition_id
+	game_state.planet_name = "Depth Probe"
+	game_state.planet_id = "depth-probe-%s" % definition_id
+	surface = load("res://surface/PlanetSurface.tscn").instantiate()
+	surface.get_node("SurfaceController").fade_duration = 0.0
+	root.add_child(surface)
+
+
+# z_index is RELATIVE to the parent by default, so a node's real depth is the
+# running sum up the chain - reading z_index alone would call a +1 child of a
+# -1 parent "behind", which it is not.
+func _effective_z(node: CanvasItem) -> int:
+	var total: int = 0
+	var current: Node = node
+	while current is CanvasItem:
+		total += (current as CanvasItem).z_index
+		if not (current as CanvasItem).z_as_relative:
+			break
+		current = current.get_parent()
+	return total
+
+
+func _drawing_descendants(node: Node, found: Array) -> Array:
+	for child in node.get_children():
+		# Enemies are spawned under SurfaceFeatures by the planet's setup
+		# markers, but they are combatants, not scenery: they share the
+		# fleet's plane on purpose, the same way they do in space. Only the
+		# ground the fight happens on has to stay behind it.
+		if child.is_in_group("enemies"):
+			continue
+		# has_method("_draw") is true only where a script defines one, which
+		# is exactly "this node puts pixels on the surface". Plain Node2D
+		# hosts that only spawn their children draw nothing themselves and
+		# are free to sit at 0 - the rocky surface's RockField is one.
+		if child is CanvasItem and child.has_method("_draw"):
+			found.append(child)
+		_drawing_descendants(child, found)
+	return found
+
+
+func _process(delta: float) -> bool:
+	elapsed += delta
+	if elapsed < 0.35:
+		return false
+	elapsed = 0.0
+
+	var fleet: Node2D = surface.get_node("Fleet")
+	if fleet.ships.is_empty():
+		return _fail("no ships spawned, so there is nothing to be in front of")
+	var ship_z: int = _effective_z(fleet.ships[0])
+	if ship_z != 0:
+		# C. The comparison below is "behind the ships", and the ships being
+		# at 0 is what makes "negative" mean that.
+		return _fail("ships are at z=%d, not 0 - this test's premise moved" % ship_z)
+
+	var features: Array = _drawing_descendants(surface.get_node("SurfaceFeatures"), [])
+	if features.is_empty():
+		return _fail("%s has no drawing surface features to check" % game_state.planet_definition_id)
+	for feature in features:
+		var z: int = _effective_z(feature)
+		if z >= ship_z:
+			return _fail("%s on %s draws at z=%d, in front of the fleet at z=%d" % [
+				feature.name, game_state.planet_definition_id, z, ship_z
+			])
+	print("%s: %d drawing features, all behind the fleet" % [
+		game_state.planet_definition_id, features.size()
+	])
+	checked += features.size()
+
+	if planets.is_empty():
+		print("TEST_SURFACE_FEATURE_DEPTH: PASS (%d features checked)" % checked)
+		quit(0)
+		return true
+	_load_next()
+	return false
+
+
+func _fail(message: String) -> bool:
+	print("TEST_SURFACE_FEATURE_DEPTH: FAIL (%s)" % message)
+	quit(1)
+	return true
diff --git a/tests/test_surface_feature_depth.gd.uid b/tests/test_surface_feature_depth.gd.uid
new file mode 100644
index 0000000..2ad8ca7
--- /dev/null
+++ b/tests/test_surface_feature_depth.gd.uid
@@ -0,0 +1 @@
+uid://c2jc6arbx322h